To be civilized and human is to care for others - empathy is a mark of one’s humanity. The main character thought he wanted to live among the animals but his concern for them - his desire to help them - reveals that he was never capable of that. His concern for others comes from the same place as his anxiety over how his coworkers perceive him. People are social, connections are important, and we understand ourselves through our relations to others. Seeking to please others and fearing their disapproval are human traits, which is why the mountain folk find such behaviour so odd.
To be accepted by the mountain means to throw away all of that. Carnivores can eat other animals without feeling any worse about themselves. That’s why he was so casually devoured and why this act signified the transformation of the cat into a feral animal - for him it was an entirely amoral act. We find it jarring as people only because his ability to speak and behave in human fashion makes us want to frame his actions in human ethical terms - it’s a clever narrative device!