Possession or alt personalities question

Movie was great, haven't felt that unsettled in a long time. I've seen the interviews where they state that it wasn't a possession. But if it wasn't a possession, how do we explain all of the super weird things that she does that don't seem to have to do with the concept of obsession. For example:
- making an altar for the dead cat
- cooking the cat and feeding it to him
- walking in a weird possessed way in certain scenes
- skittering across the room with the flowers in front of her face

All of the above seem like the type of thing that someone who doesn't know what it means to be human would do, certain scenarios seeming like a form of torment.

A lot of the other scenes can be explained by the concept of obsession, like watching him sleep, waiting at the door and soiling herself until he gets home, having a meltdown when he wants to go and hang out with other friends or when he had the opportunity to kiss Sarah, etc. But I don't get how some of the other super erratic behavior would be explained by anything other than some kind of possession. Probably overthinking it, but everything just felt so intentionally written that it's hard to not wonder. Either way, really hope they expand on this universe!