Kafka is evil in its purest form. I don't know what caused it (there are many possibilities... her tragic past, her communication with malevolent aliens, etc). I think the reason why she manages to be so positive all the time is that she's so twisted that everything looks straight to her.
Next most evil in line is Chiri, of course. But she's a few steps down the ladder.
I dont know if she thinks she's evil, but there's definitely something not right with her. Did you notice her in the Zan ending, just standing there in a kimono in front of the school at night time? Something has been messing with your egg salad if that's your night time activity of choice.