why did the friendly, kind, saving person, in love (i think) with xiao yan girl start cont...

1 she was bitter about people close to her dying and probably didnt care which side she was on. Or maybe they evil clan (forgot name) promised her a cure?
2nd she was ashamed? Or maybe she didnt want to guilt xiao yan

i know she is bitter, but still, for her to start killing everybody... and like, how she reacted after xiao yan got poisoned: "let's kill everybody so you won't die alone". i don't get it...
she should listen to what xiao yan wants...