Some are young poor boys charged with crimes and sentenced to a life of service with the Night's Watch, who take whomever they can train. Others are squires to this lord or that knight, learning the crafts of war so they might be knights themselves in time.

As for the others... I do not speak for grown, armed men who slaughter defenseless children in the streets like animals, but I am told when a man's blood is up he might be capable of anything. When a city is sacked perhaps any living creature looks an enemy to a man with a sword.