Personally, I hate the guy, along with his characterization. And if you're a die-hard One Piece fan, please don't make a fool of yourself by lynching and/or trolling me... yet. At least let me explain myself concisely.
1.) This is NOT because he killed Erza in Death Battle. I came to this conclusion after I did reseacrh on Zoro's character (research that excluded watching/reading One Piece, which I'll explain shortly). I conducted this research shortly after he was announced as Erza's opponent, and came to my conclusion long before the actual battle aired.
2.) Before you tell me to piss off and just watch the show before sharing my opinions, my answer is no. Because watching any anime will inevitably shape your view of its characters based on the desires of the writers. Meaning if they want you to like Zoro despite his unlikable character, then doggone it they'll get you to like him. How do you think so many people became fans of Death Note, or Code Geass, or (fill in the blank with any dark but enjoyable anime)?
3.) I welcome civilly presented, logically sound counter arguments. But any and all flame/fanboy riots/etc. will be dully ignored.
So, here is my "Reason You Suck" Speech for Roronoa Zoro, broken down as thoroughly and concisely as I can manage. In a nutshell, his character archetype is by far one of the worst I have ever seen in the history of fiction, for the reasons I will be listing here. This archetype will be defined by a formula of tropes, and then explained below. The tropes this archetype typically has are added, the tropes they need are subtracted, and tropes that often coincide are multiplied by each other:
(Informed Flaw x Mr. Vice Guy) + (Showy Invincible Hero x Smug Super) + (Offscreen Inertia x Designated Hero) + (Karma Houdini x Worf had the Flu) - Break the Haughty - Brought down to Badass - Took a Level in Kindness - Reality Ensues = (what I like to call) The Insufferable Badass.
Here we go.
1.) This first one is the least important, so I'll get it out of the way first. This character has some kind of otherwise-debilitating flaw that would seem off-putting in real life (in Zoro's case, it's sleeping a lot and drinking even more), and yet the plot pretty much ignores it.
Comments: On one hand, this isn't one that I have as much disdain for, and it's typically one of the marks of a Mary Sue. And not all Insufferable Badasses have this trait. But I can safely say that it's the most common similarity between Insufferable Badasses that isn't directly connected to what I find unlikable about them. Think of it as their mainstream calling card.
Be that as it may, I find that it adds insult to injury when combined with the cluster-tropes below, simply because it shouldn't be something any realistic character could feasibly get away with.
2.) The character is portrayed as such a huge badass that there is no dramatic tension over the possibility of them losing. On top of that, the character is aware of this, and whenever they're in their element, they smugly flaunt it to whoever challenges them.
Comments: Zoro has this one written all over him. Granted, he saves the Smug Super behavior for his enemies, but the intent is the same. Unless this kind of behavior is played for laughs like with Bugs Bunny or Perry the Platypus, there's not much that can really justify it. The Rule of Cool does keep it from getting boring, but it doesn't detract from how poorly it reflects on the character's development.
3.) This is by far the one that I find most unlikable. Despite the fact that the character is established as one of the protagonists, their behavior very frequently flies in the face of their supposed heroism. And while many main characters like Deadpool are portrayed as anti-heroes or even villains (e.g. Light Yagami), this cannot be done carelessly. Deadpool manages by following the Rule of Funny (which, when compared to the Rule of Cool, is harder to mess up and doesn't get old as quickly or easily). Light pulls it off by being an excellent catalyst for the Rule of Drama. But Zoro... I'll get to him in a second.
However, the insulting part of this comes from the fact that the plot never gives any indication that the character's behavior is improving for the better, implying that (worst of all) their more despicable actions continue to go unacknowledged. The key here is that the Insufferable Badass fails at compensating for this unrelatability, or just does it poorly.
Comments: I could go on for hours about how much this pattern pisses me off when I see it in a character. But I'll just stick to the point I'm trying to make. Zoro fits the bill for this rather uniquely. On one hand, he completely averts it around his friends and those who need and deserve his help. In these situations, he acts like more of a Jerk with a Heart of Gold. But on the other hand, Zoro's favoritism towards his crew is a cookie cutter staple of shounen anime. And even though he does do heroic things for people, the fact that he treats others with brutality and indifference paints Zoro as more of a Hero with an F in Good than anything else. Can this kind of behavior be adequately justified? Sure. Is it justified in Zoro's case? Without a doubt. Does that justification make his character any more respectable? No... not really. In fact, not in the slightest. All kinds of anime characters (*cough* ...ERZA... *cough*) go through the same stuff and maintain a sense of heroic integrity. And the main thing that keeps Zoro likable, the Rule of Cool, has a fairly limited shelf life in the minds of those who want more than just action for its own sake (meaning that Michael Bay fans would say that the Rule of Cool never gets old). So Zoro really has no excuse. C'mon, Zoro. Get over yourself and stop being such a douche. It's not rocket science... or navigating.
4.) This one ties everything together and makes it all a thousand times worse. The combined pattern of the Karma Houdini and Worf had the Flu tropes makes this stereotype into the holistic garbage I am convinced that it is. In the midst of all the things listed above, the character dodges any kind of comeuppance thrown their way, particularly when they do something notably unlikable. Not only that, but the plot goes out of its way to disassociate any of the character's equalizing defeats with the notion that they might have gotten knocked down a peg. This last pattern once again makes everything else mentioned above much worse in context.
-When applied to the Informed Flaw/Mr. Vice Guy pattern, it shows that the character's flaws are deliberately being downplayed and ignored, adding to how contrived the durability of their Plot Armor is.
-When this is applied to the Showy Invincible Hero/Smug Super pattern, it leads me to believe that Zoro (or any Insufferable Badass) could potentially be a full-blown Mary Sue (albeit an incredibly jerky one). After all, not only is the hero flaunting their invincibility, but that invincibility is actively protected by the plot. The fact that the narrative goes out of its way to ignore the struggles the hero faces in favor of how awesome they are... that is precisely why I'm not impressed by Zoro's amazing qualities or feats.
One Piece Fanboy STEREOTYPE: ZORO CHOPPED A MOUNTAIN IN HALF!! *fangasm*
Me: Uck, so? It's not like the plot was trying to make that significant to the story. It's just absurd awesomeness for the sake of absurd awesomeness. BORRRING! If I wanted to see that, I would watch Man of Steel or one of Michael Bay's films.
-But what this pattern implies about the Offscreen Inertia/Designated Hero pattern is arguably the worst of the worst, particularly with regards to how Karma Houdini applies. The story is basically saying that we can never expect anything to change about the Insufferable Badass in a significant way. Whether they are loved or hated by anyone (in-universe or in real life), their character arc is pretty much going nowhere. Not only that, but they will never be definitively payed back for the ways in which they've acted like a jerkass. In fact, I guess you could say that Offscreen Inertia, Karma Houdini and Designated Hero form the unholy trinity of this stereotype. After all, you've got a character with a.) unlikable behavior that flies in the face of their heroic characterization (free of any notice or justification), b.) the ability to evade any comeuppance for the unlikable things they do, and c.) a viscous cycle that will keep their awful character in place indefinitely. )<>
That... is one of the most atrocious characters imaginable. That is a character I want to see suffer a Humiliation Conga. That is a character I will never have any respect for, for as long as I live.
*sighs* Sorry, that got a little heated. But rage aside, my point stands. And the aforementioned apostasy is in no way helped by the fact that Worf had the Flu. It just means that all the other characters can't do jack squat about the Insufferable Badass.
It doesn't have to be this way. It really doesn't. I've actually seen this character type done well on multiple occasions.
Qrow Branwen from RWBY, however, is the best example. After all, almost all of the above tropes are explained/justifies by Qrow's Semblance and the rest aren't fulfilled at all (SPOILERS UP TO SEASON FOUR, EPISODE EIGHT OF RWBY): He brings bad luck to everyone and everything in his vicinity. Indiscriminately. The fact that he constantly brings misfortune to his foes justifies his supposed invincibility. And if he uses it for good, then why not rub it in his opponent's face (as long as you don't give it away explicitly)? And Qrow's personality is rounded out enough that his ass-kicking doesn't crowd out the rest of his characterization (Zoro's character may be well-written, but it's also a one-note song of occasionally-subverted stoicism, crowded out by his badassery). But what's important in Qrow's case is that his characterization actually fits the bill for a well-written anti-hero. He does good often and sincerely enough that it doesn't seem forced or reluctant (Zoro may do the same thing, but he's not really a heroic anti-hero. His deeds of heroism are few and far between, and they're far exceeded by his indifference towards most people and his blood-lust for his enemies). Also, Qrow makes a visible effort to atone for how much of a jerk he can be at times. He helps out whenever he can and treats his friends and family like friends and family (doting on his niece Ruby especially)... But he still has the courtesy to keep his distance lest his Semblance bring harm to those he cares about.
But most impressively of all, Qrow balances his good traits out with his less likable tendencies. He's still a hard-headed prick who enjoys pushing people's buttons and refuses to knuckle under for anyone but Ozpin. And he's a drunkard who lets his pants do the thinking when there's women involved (at least when the head on his shoulders gives him the OK; like when he chooses to hear whatever intel his sister Raven has to share, instead of immediately hitting on the waitress who had just flirted with him). The kicker is that it makes sense without any kind of contrived reasoning. Qrow is not only forced to bring misfortune to all those in his wake. He also has to choose between keeping anyone he cares about at arm's length or subjecting them to the same misfortune as his foes. That would make pretty much anybody cynical, especially a hard-working Huntsman like Qrow. So that explains his being a jerkass. Not only that, but it makes perfect sense that he would turn to sex and alcohol as a way to cope. And it's not like Qrow is taking all the good luck for himself. His team fell apart because of his sister, and by the end of Season 4 Episode 8 he's confirmed to be poisoned. All that's really left is something to show that no, Worf does not have the flu. Across the board, Qrow fits this stereotype in a way that I wish could be done for every Insufferable Badass, up to and including Zoro. Before the reveal of his Semblance I hated Qrow for all the same reasons I hate Zoro. But RWBY did a great job of putting his characterization into the proper perspective.
But, as he is now, I cannot respect Roronoa Zoro to any extent, nor any other character who handles the above tropes sloppily. Everyone has something in fiction that they just can't be accepting of. For me, the Insufferable Badass is that thing.