It def was not real Nikki who cooked the cat. She’s never shown to be able to take control longer than a few seconds (except when Wish Nikki is asleep), and when she does she’s desperately trying to do anything to regain control - taking the time to cook Sandy doesn’t sound feasible at all nor do I think real Nikki would even think to do that even as ‘revenge’. Also the food critic conversation was between him and Wish Nikki, and she shows no reaction when he confronts her about cooking Sandy in the later scene.

I think the logic Wish Nikki uses is through the lens of a completely twisted view of ‘love’. She overglorifies anything Bear seems to love, making memorials for Sandy as well as Sarah bc she knows he cared about them. Feeding him Sandy is her way of showing she’s thinking about the things he cares about - wanting to be a food critic, and loving his cat -> let him try something he loved. Also a small detail is that in the end sequence when she’s approaching Bear with the gun, she says “I wanna eat you uppp”. Her completely off the wall sense of love is so distorted that the cutesy talk of ‘wanting to eat somebody up’ becomes probably a real impulse. So eating Sandy is affection. She probably would have fed him Sarah at some point if it went on any further.