Being an ex-marine doesn't mean shit in terms of how you'd handle yourself in a situation where someone was being assaulted. Some of the most mentally fucked up assholes who've ever walked the Earth have been in the marines.
Knowing the mechanics of pointing, aiming and firing a weapon is a completely different matter than how cool, calm, collected and aware one remains in the heat of the moment.
The potato peeling and floor mopping skills you acquired in the military wouldn't have prepared you for any of that.
If you could even summon the cajones to involve yourself in someone else's emergency situation, you'd probably end up spraying stray bullets everywhere but at the bad guy.