The thing is...the events that happened in her past drove her to hate humans...and when she found trust in one, she felt betrayed after what he did. Mankind only showed it's bad side to her. She isn't evil, just misguided, depending on what perspective you look at it from. Were her actions justified? Was she wrong in killing? Did the humans deserve it? This all depends on how you look at the character, but describing her as "evil" is a little too easy for the depth of the series. Elfen Lied isn't your good guy, bad guy type anime.