Mother’s Milk, or MM as he prefers to be called, narrates to Hughie Campbell how Billy Butcher and Greg Mallory recruited him to join the Boys. MM signed up for the Army and ended up in the Rangers. During a boxing tournament, he was being beaten badly by a boxer named Fucker John. Just when he was about to lose, MM’s powers activated and he ended up punching his opponent’s head off. He was sent to the brig where Butcher and Mallory found him.
Butcher showed him the file that Vought has on his family history, explaining that Vought keeps tabs on everyone they’ve encountered that might show powers. Offering him a chance to get back at Vought, MM joined the Boys.
In the tv show, MM mentioned that Butcher found him in the army but not much was said on the specifics. I doubt he killed a boxer since he MM doesn’t have powers on the show.
From – The Boys #35