If we saw everything "evil" she has done from her PoV we would feel that she is crazy and not really evil. Looking at her from somebody elses PoV-- say somebody more people can relate to such as a protagonist-- we would see her as evil.

Cersei is still believable as a person. There are no Good Guys or Bad Guys in life, simply many people with differing values and views of the world who act upon the world in different ways from our own. Think of Ozymandias and tell me if he was a good guy or a bad guy.