Sniper Wolf. By now it's practically a cliche to have one or more MGS boss be a victim of third-world wars or some other tragedy, but in the first one Wolf was the only one who didn't seem to have a choice in the matter but to become an assassin (well Mantis too but it was his ability that messed him up and he still had much more control over events than Wolf).
'Siren, gunshots, screams. They were my lullabies. The governments of the world turned a blind eye to our misery.'
The kind of thing that makes you vow 'never again' except it goes on in RL and in the series (seeing as all four of the Beauty & Beast unit were similarly victimized by Patriot-created wars).