Someone explain this to me. Isnt he just manipulating her and gas lighting her and etc?

Yes he is, he’s also activity(slightly) altering her memories

Just some kind of eldritch monster games, you know how it is ¯_(ツ)_/¯

So he is trash. Got it.

I dont actually its why i asked.

From his POV, he's like a human surrounded by nothing but talking ants, and getting insanely bored. Then one day one of the ants (our mc) happens to notice he's not just another ant, so he gets...curious...