You are thinking a bit narrowly about the impact that liability insurance would have. Sandy Hook for instance - Lanza's mother likely would have been priced out of the ownership of weapons given Adams mental state.
In this incident, I would not presume that the perp would have been able to afford or been underwritten for gun liability insurance.
Again, cost is rarely an issue as personal insurance companies never require 100% of the premium up front. As for the underwriting issue, there will always be a carrier that will issue someone a policy. Progressive is a great example of that with respects to auto insurance. No matter how many DUIs, no matter how many traffic violations, no matter how many accidents...they will absolutely underwrite anyone.
Plus, the narrow thinking is the assumption that any insurance company on the planet is going to write a policy centered around personal gun usage.
It simply is not feasible.