I think, while not evil, Doc Ock was clearly mentally unstable prior to the accident. The accident was, however, the trigger. It's the same way with most mental illnesses that you carry it for long with some signs of it now and then, until there's a trigger that surfaces it entirely.
I am aware of the fact that the accident damaged his brain, so I'm not arguing that. I still think the A.I. chip is a bad way to interprete that though, and I'm still very disappointed that they went with that. The same way I'm disappointed that he didn't feel like a "Masterplanner". Also the same way I'm disappointed we didn't get to see Connor's family in ASM, or Lizard not being feral enough.
And yes, you can be mentally and emotionally unstable while still being a great scientist.