He never shot a child
The main antagonist for the movie never had any interactions with him at all (the other sniper)
He was interested in joining the military out of school, it wasn't seeing terrorist attacks that spurred him on
He was rejected by the Navy initially
He didn't decide not to re-enlist because he couldn't handle it psychologically, it was because his wife told him she'd leave him if he didn't (in the movie she does this but he re-enlists anyway)
The whole thing with the weapons cache under the guys floorboards never happened
He joined the military in his early 20's, not at 30
The whole Butcher thing was largely made up
Maybe I knew too much about the guy going into the movie but I couldn't take it seriously given the amount of main plot points they'd made up or exaggerated.