Willard (1971)

A social misfit, Willard Stiles is made fun of by his co-workers and being squeezed out of the company that was founded by his deceased father by his tyrannical, foul-mouthed boss, who was also his father's former business partner. His only friends are two rats that he has raised at home, Ben and Socrates (and their increasing number of friends). However, when Socrates is killed at work by his boss, he goes on a rampage using Ben and the rest of his rats to attack those who have been tormenting him.