although I'm not a lawyer, the law generally holds that you have a right to self defense, but only in equal measure of the threat against you. If someone tries to punch you in the face, you can punch them back to stop them - not stab them. If they are taking after you with an axe or bowie knife or something, you have a reasonable fear of your life such that to stop it you can employ deadly force. You are required to somehow know the instant that the threat is over, so if you do stab the would be axe murderer once and he drops the axe and falls over, you can't do anything else. If you do some Kali move and create three holes real fast in the guy... they might hang you for those two other holes.
That being said, the best weapon is your brain. Try not lurking in Las Vegas back alleys at night, for instance.
If you do make a personal decision to carry a knife for self defense, read the local laws very thoroughly!
And finally - get something with a front guard, in my opinion. If you have a design like the Military and you do stab said axe murderer and you hit bone... you are going to slide right up that thing onto the blade. I have one, and its a great utility knife... but I don't think its designed well as a weapon. Just theoretically, of course. I do not advocate or advise violence in any fashion and would prefer all our good citizens to avoid any armed encounters!