I think being a ghost has something to do with it. When you're free from human/social chains combined with only having rare guests, I can totally see the "I don't give a shit" mentality kick in.
Plus, she seems to think her ghost self is not her real body. I really can't relate to any out-of-body experiences though so I can't comment much on that. xD
Amnesia hasn't been focused on horror so far as you've said, but those flashbacks of Yuuko's death were pretty horrific enough.. I can see this getting dark (or at least 50% dark).