SPOILER:



















His walk down the hallway at the end, with the bloody footprints, was similar to his walk down the hallway as he left her apartment. Plus we never see her again in the movie. But then she didn't outright wrong him like the other people he killed, so he didn't really have a reason to kill her. It didn't fit with his twisted moral code.

But I mean if the director himself said he didn't intend it that way, eh, it's probably not worth debating.