>>721194525 (OP)
Abused by humans so badly that she got an unyielding lust for absolute power and control so she couldn't be hurt by anyone anymore, and when she tried to make a power grab that resulted in her death because Asriel's too much of a good boy to let revenge happen, she was forced into seething dormancy.
It's only when (You) pilot Frisk to go on a murder spree that she finds ample opportunity to rebound, and thus supports your endeavors until she gets what she wants out of it, leaving you in the dust of your own actions in the process.