When bullies grow up, we imagine they grow up to be a lot like Judson. Instead of pulling the wings off of flies, they're beating soldiers to death.
He doesn't start off seeming like such a bad guy, though. Sgt. Judson (Ernest Borgnine) likes to relax by loudly and badly playing the piano. What's wrong with that? Not necessarily anything—unless you're doing it loudly in a social club where people are trying to relax.
In a rash moment, Angelo Maggio berates Judson for his piano playing—spurring a conflict that won't end well for anyone. Judson is in charge of the stockade (i.e. jail) at the military base: he's not a good person to make enemies with.
Unfortunately, this enemy-ship escalates when Judson creepily admires a photo of Maggio's sister. It almost erupts into a knife fight, until Sgt. Warden makes peace. But Warden can't defuse the personal hatred. So when Maggio goes AWOL from his post, gets court-martialed, and is sent to the stockade, Judson has virtual free reign to beat the snot out of him… which he does, leaving bruises where they won't show. Classy dude.
Maggio escapes the stockade and dies of his injuries, leaving Prewitt to seek revenge. However, when Prewitt confronts Judson—intending to give him a presumably non-lethal beating—it quickly escalates into a full on Sharks vs. Jets-style knife fight. Prewitt kills Judson and Judson severely wounds Prewitt.
The moral of this story? Don't criticize sociopathic rageaholics' piano skills.