Oh isn't it obvious? It was the member of his mouse-school clique - they dared him to go into the house of a known cat and spend the night. If he made it through the night without running scared before morning, then he'd get to be top mouse.

Mr. Katz was an innocent victim too.. he was just doing as nature intened and the mouse clique knew that. you see they hated the victim and wanted to get rid of him without pointing the blame at themselves.

Amatures.