SPOILER
its not a theory but it was mentionned in the game (lil reminder that theres 2 endings). althought,Idk how to answer the 2 last parts. idk if the cat is supposed to be ryuk from death note and if theres a specific reason on why zilas just didnt randomly die. but if i had to give u an answer, ig its prob how the cats "business" works. the death of someone is probably supposed to happen because of something (like the hate zilas got which lead him to suicide according to one of the endings).