난 is a short form of 나는
나는 is composed of
는 is a topic marker
내가 is composed of
가 is a subjective particle.
난 내가 생각해도 나쁜놈이다
Even if I think of it, I am a bad guy.
= Even in my thought, I am a bad guy.
= I also agree that I am a bad guy.
도 is a emphasizing particle which means "even,also"