Malefactor

Definition: a criminal; one who commits an offense against another

This is exact opposite of “benefactor.”

The root of the word is “mal,” which is Latin for “evil.” It’s the same root from which we get “malevolent.”

“factor” means “to do.”

Why I Looked It Up

I had heard the word, of course, but wanted to clarify the meaning the etymology.

From Team of Rivals, William Seward is defending a man for murder whom we truly believes to be mentally ill:

I am […] shocked beyond the power of expression, at the scene I have witnessed here of trying a maniac as a malefactor.