People are crazy.

Remember law school? I do not. I do remember my classes on US constitutionality.

I have never heard a good argument against weapons. I mainly believe that it is a question of personal responsibility. I have the ability to make explosives, but I chose not too. I have the ability to make nerve gas, but I chose not to. I just do not believe that a few murderous outliers represent the American populous as whole.
I had to dig in the back of my closet for my essay on this, but I found it. Marguerite Anne Bowers was killed with a blade by Thomas Vanda who was a "schizophrenic in remission". A subset of the ruling follows:

Bowers v. Devito
UNITED STATES COURT OF APPEALS FOR THE SEVENTH CIRCUIT

But there is no constitutional right to be protected by the state against being murdered by criminals or madmen. It is monstrous if the state fails to protect its residents against such predators but it does not violate the due process clause of the Fourteenth Amendment or, we suppose, any other provision of the Constitution. The Constitution is a charter of negative liberties; it tells the state to let people alone; it does not require the federal government or the state to provide services, even so elementary a service as maintaining law and order. Discrimination in providing protection against private violence could of course violate the equal protection clause of the Fourteenth Amendment. But that is not alleged here. All that is alleged is a failure to protect Miss Bowers and others like her from a dangerous madman, and as the State of Illinois has no federal constitutional duty to provide such protection its failure to do so is not actionable under section 1983.

In summary, Americans are required to protect themselves because the US government is not required to do so.
I believe that you cannot take the behavior of crazy individuals to account, because they are crazy. If someone hits a school bus while drunk, should we ban all cars? How about kitchen knives?
If one is feeling criminal, by definition, they will not follow the law. I'm just glad that most crazy people are not well enough to get interested in chemistry.