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. 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.
Thought I was going crazy because I was sure Crispin Glover was in this.
I didn’t realize there was an earlier version from 1971. Glover was in the 2003 remake.
The 1971 original even has a sequel, it's called "Ben". They are both decent movies IMO, though I haven't checked out the remake yet.