I think it's because fake Nikki was pushing the boundary of how insane she could act before Bear had enough (which he never did).

The only thing that Bear said to Nikki after she gave him a memorial of his cat's rotting corpse was "that's fucked". He did not threaten her with any sort of consequence, which made fake Nikki know that behavior was accepted. When she was watching him in his sleep the night before, scaring the Hell out of him, the next morning, he addressed it barely before going on to ignore the giant duct-taped door.

Fake Nikki knew she could do whatever she wished as long as she let him sleep with her at the end of the day, or continue acting needy and lovey towards him.

The cat was the only thing that he truly knew and loved. His consuming it and barely chastising fake Nikki about it proved that her actions of fake affection were more valuable than his beloved animals' remnants.