You need to read more onto it, Historia's "fake" personality is that of a Saint who live for others. That's what Christa was about. She would do anything to please other people because that was for her the only way to "survive".
She wanted to "kill herself" because she was at her limit. Either she gave up at being Christa (living for others) and get killed by others, or kill herself as Christa. She chose the latter because she wouldn't feel "hated" that way.
Ymir was someone who pretended to be someone else for the sake of others because it was the only way for her to survive and be happy. But in the end, she suffered like crazy for it.
She stopped being a tool but she still kept her fake name for bullshit reasons.
Seeing Christa reminded her of her old retarded self. And she wanted her to stop before that shit backfires on her like it did for Ymir back then.