Well, technically, the guy was a POW that was tortured after a few day of it, he cracked and developed split personalities because of it.

One personality is his carefree, immature side, and the other his mature, stuck up side. They both got his military training, but the British guy thinks guns are "simply brutish" and prefers to let others fight for him if he can. Of course he's too snobbish to walk around in blood-soaked clothes, so expect him to want to change after every battle, shave every morning, and be generally "too good" to stoop to burial detail. The other guy figures this is his time to cut loose and have fun. Also, as the movie says, he wants to bug out and nuke the sight from orbit, but he's not so crazy that he'd knowingly kill allies. Neither remembers life before they split, except for the occasional dream or nightmare.

Over the course of the RP (if he lasts that long) the two will merge and he'll eventually become a normal guy.