The cat killers, 43-year-old twins whose names were withheld, went on a rampage because they believed themselves "aliens" whose spaceship departed the Earth to escape the end of the world Dec 21, but left them stranded, Novokuznetsk police said.
The brothers tortured the cat to death in hopes of somehow claiming two of its lives for themselves.
The doom mongers expected the animal to come back to life after its first death, but it never happened, police said.
The duo, countryside residents who came over to Novokuznetsk to visit their parents, barricaded themselves in the apartment and broke all mirrors and TV sets because "demons lived there".
When their elderly mother tried to leave the place, one of them shoved her back into the room so hard that she fell and broke a leg, police said.
The noise and screams prompted neighbours to alert police and emergency services, whose operatives broke into the apartment and detained the rampaging "aliens".
The cat killers would suffer no criminal penalties because of their mental problems, police said. The duo was confined to a psychiatric hospital.