The job of the police is NOT to protect each person. Their job is to protect "the public". As a whole, not generally on an individual level. That is the point; you CANNOT prevent crime. However you CAN discourage it from happening by making the consequences so high, no one dares do it. How is it difficult for you to understand that there WILL ALWAYS BE CRIME? That's a simple fact; it is impossible to eliminmate crime, unless human nature is naturally good. Oh wait, look at all the crime in the world. Guess humanity is kinda lousy in that regard, so...crime isn't going away any time soon. However rather than hiding my head under my blankets, I choose to do something about it. Certainly, there might come a time when I may be faced by an armed man with murderous intent. And should that ever happen, I will be prepared to defend myself.
And, as our friend with a wooden leg has so aptly explained, nobody is talking about giving the mentally ill guns. You're the one bringing that up, because that's how you want to make others look bad by painting them as horrible people who want to arm those with mental disorders.
So the prerequisite for owning a gun is that it MUST be for self-defense? Very well; that means no one should be able to buy a Ferrari, because you may only own a car as a means of transport and a Ferrari is overkill. I figure you'll probably discount that argument as ridiculous, but it's the same one; if you can tell a person they may not have a certain type of gun because they don't "need" it, why should we be allowed to purchase anything we don't "need"?
And the fact you are still ignoring is that it doesn't matter if full-auto guns are legal; hop over to Mexico and buy one! Ever head of Prohibition? Didn't work out so well, now, did it? Marijuana is also illegal in many places, as are Cocaine and methamphetemines. Yet they're still used. Hmm. Guess a lot of people obey the law, don't they?
Focus should not be on the tools used to commit a crime; focus should rather be on making the punishment fit the crime.