But I know I can't write for an evil main character well, I think.
Writing an evil character is like writing a normal "good" one.
Except, the character is well... evil.
It all depends on the chara, actually.
For dialogue:
Is the character a rich snob?
If so, make him talk all distiguished and [parasitic bomb].
Is the character a punk?
Make him swear and [parasitic bomb].
Is the character a psychopath?
Make him talk crazy, he can either swear or talk like rich snob or something completely different. Or a mix of the two (or more). It depends on how crazy he is and in what way.
This is a pretty crappy and rushed advice, but I think it gets the important point across.
Just try to think of what/who the evil guy is. How he would act in such and such situations. And so on.