Ooh. A place to express my views. *drools*

SPOILERS THROUGHOUT. You have been warned.

Ok, here's the rub: Vayne's motivations are quite justified. It's just that his methods are screwed up. All he really wanted was to free humanity from the control of the Occurians. He just had to go through a lot of obstacles to accomplish this goal, and he certainly didn't mess around. That's the issue: whether his motivations or his methods determine his 'evilness'. I personally think it's his methods. Slaughtering a community of refugees is definitely an evil act. Framing someone for killing a king is an evil act. Actions, not purposes, define good and evil.

However, there is one variable: Venat. It is completely an open question as to whether Venat is evil or not. All that's definite is that he opposes the Occurians' mucking about with the mortal realm AND tapping into his power drives you insane. It's rather obvious that Venat is the source of Vayne's motivation, but whether it was Venat's influence or Vayne's own decision to commit evil acts is open to interpretation.

Basically, it all depends on your definition of evil. I think he was evil because I define evil by actions taken. You might think differently. Any interpretation is equally valid.