if i were to make an evil character i should not make him evil for the sake of it.
He shouldn't be evil in his own eyes.
He should fight for a purpose. (That he believes is good but is really evil)
While the heroes might call him 'Evil Demon Knight Of Terror' he shouldn't, so he should call himself by his own name.
I should write a topic or two to explain how he has become what he's become.
Those (1) 2 topics should start out innocently enough and explain how he became twisted and evil.
And he should have his own opinion on those who oppose him. (Either he thinks highly of them and regrets that they're in his way, or he holds a huge grudge against them.)