Would you have a hard time killing someone if they attacked you or a loved one?

It doesn't need to be gory or gross. Say a large, intimidating figure attacked your spouse or parents, would it be hard for you to shoot them in the head and kill them? My quick answer is "no" but I'm interested what other people say and why. My girlfriend was horrified when, after watching a movie where this scenario ocurred I remarked something like, "Boy, that was dumb. I would've just shot him in the face." Granted, not a romantic thing to say on date night, but it slipped out. Do you think you could do it, no problem?