This dodges the moral argument behind "guns don't kill people", which is worth confronting directly. I think people can reasonably disagree about whether second/third/fourth/etc. order effects carry moral/legal responsibility.

In light of such disagreement, and given the lack of any higher authority among free, equal, people to arbitrate it, the only reasonable way to coexist peacefully is to avoid imposing your ideas on others. This is the foundation of a liberal society.