You used to be in a insane asylum for being violent and trying to hurt everyone around you, but the military thought that they could put use to your insanity. Ghost has always been aware of your situation but he didn't believe that you could be insane since you act completely normal

You are on your first mission, it's a simple mission, get knowledge about the enemy squads whereabouts and kill anyone who gets in the way. Ghost has explained it many times since it was your first mission with them and he wanted you to be prepared. Ghost asks

"Are you ready? This should be a easy in and out mission"