Ok first off Ozipin was confirmed to be the original Wizard from the tale of the 4 Maidens.
In the present Ozpin is cursed because he failed to stop Salem in the past which means there was a defining event that changed the world. I think this was the creation of the GRIMM (Although writing this down the moon shattering could come into play somehow).
The tale is important here because the Wizard says he'd been chilling there for centuries. Not exactly "I'm on a mission to stop Salem's destruction of humanity behavior."
The next incarnation we have is the King of Vale (see the relevant Qrow short) who ended the great war and set up the huntsman academies. GRIMM confirmed to exist at this point.
So something happened between these 2 points that put Ozpin and Salem on their current path. I think this is what happened.
1) Ozpin Wizard is chilling then creates the maidens with a mission to spread goodwill and help humanity.
2) Regular people freak out at concept of magic and have a good old fashioned witch hunt.
3) Three of the four maidens are burnt alive. Winter surviving because she can counter fire duh.
4) Winter says screw humanity goes AWOL.
5) Wizard vs Winter; Winter wins; becomes Grimm master and Ozpin is cursed for failing.
Evidence for this is: Ozpin is the Wizard, safe to assume Salem is the same age due to their intimate knowledge of each other. Ref Volume 1 episode 1 "Perhaps victory is in the things you've long forgotten"
Volume 3 Finale "I can't wait to watch you burn" Just like my sisters did
This would fit into the sibling love themes of the show and it explains why Ozpin can't tell us the whole truth; his arrogance got Salem's family killed and started this nightmare.