[Steko]

"Psychopathic spree killings are such a low-probability, extreme event that there's almost nothing you can do, at a public policy level, to influence them. "

This is completely false. Being rare doesn't mean there aren't things that can make it even more rare. Many countries have taken steps that make these events far less likely.

[neilk]

You're right - I misspoke there. The OP was more careful, and discussed "iatrogenic" harms (an analogy to medical harms that arise from an attempt to treat a problem).

So, I'll revise what I'm saying to this: if you have a measure that you want to take that is only justified by such extreme events, it's probably not a good idea.