Dougal Thorn, 42, punched Chopper in the head before strangling him, Worthing Magistrates' Court heard.
The softly-spoken tenant "flipped" after the cat came into his house in Shoreham, West Sussex, and knocked over a vase of flowers.
But it was Chopper's bullying of his landlady's cat Barney that drove him to violence, Thorn said.