When "push comes to shove" I have a haunch that strong aikido and a 45ACP pistol may be all that is really needed in close quarters, in a street, home, or in an airplane to resist "evil."
A gun will get you in trouble in your home, on an airplane, and especially in the street. If you don't get shot yourself, just add an extra 25 years to your sentence for using a firearm in commission of whatever felony the end up getting you on. While you wouldn't want to bring a knife to a gun fight, a knife may be a better choice for personal protection. Because at least you don't have to worry about incurring firearms violations, yet it can be just as deadly.