The company said it fired Kristopher Oswald for "a violation of company policy," WXYZ-TV, Detroit, reported Thursday.
Oswald, 30, said he was taking his lunch break in the parking lot of the store in Hartland when he heard a woman screaming about 2:30 a.m. When he went to see if the woman was in danger, he was jumped by a man who punched him in the head, saying "I'm going to kill you."
The clerk said he was able to get on top of the man, but was attacked from behind by two other men. Sheriff's deputies arrived shortly and ended the fight.
Walmart fired Oswald for intervening, saying on his termination papers, "after a violation of company policy on his lunch break, it was determined to end his temporary assignment."
The company has policies against workplace violence to prevent employees from attacking co-workers or tackling shoplifters.
Even so, Oswald said he would do the same thing again.