My early guess is the killer is someone the girl knows, since a stabbing like that has to be a crime of passion. Maybe an ex-boyfriend she just broke up with, since she was acting peculiar in her opening scene, giving Nasir no real destination to drive to.
Honestly (episode 1 speculation)
it crossed my mind several times that maybe it really was Nasir and he doesn't remember. The girl was into kinky stabbing shit and maybe they took it several steps too far after taking more drugs