A couple things:
1. People never "suddenly go insane". There's always a build-up. Mental illness, whether caused by a chemical imbalance or a brain tumor, is a progressive process and it has symptoms. This fellow in AZ was a textbook case. Instead of tossing his crazy derrière in a mental institution, however, people merely observed he was crazy and "hoped" he didn't "suddenly snap". Except of course, people don't "suddenly snap", there's a build-up, which everyone witnessed, and nobody, in a position of authority or otherwise did anything about!
2. We're too politically correct. To echo an Ellen Degerneres stand-up skit, "Whatever happened to the people who were just crazy?" Today, we label people with fanciful terms, while ignoring the fact that in many ways, those terms are merely suborders/classes/families, etc. of the same kingdom, crazy people. We don't want to offend people's sensibilities. Well, if someone is a danger to society, I say let's call a duck a duck and get that person committed (or get them "help" or whatever the PC term is today).
3. Banning objects will do nothing to deter a crazy person fixated on harming others. The issue is that someone thinks that murder is acceptable. It doesn't matter how they kill their victim or with what implement, it's that they kill someone in general.