It hasn't been fully explained, and in fact Kirin has actively opposed Sensei's attempts to figure her out and get more information about her motivations.Has it ever been explained why Kirin behaves the way she does? I'm assuming a good deal of it has to do with her desire to be different from her sister but that can't be the whole story can it?
I'm sure we'll get more information eventually about why she decided that she wanted to be a sociopath even though she didn't start out as one.