Willard (1971)

A social misfit, Willard is made fun of by his co-workers, and squeezed out of the company started by his deceased father by his boss. His only friends are a couple of rats he raised at home, Ben and Socrates. (And their increasing number of friends) However, when one of them is killed at work, he goes on a rampage using his rats to attack those who have been tormenting him.