I thought this thread was about as productive as any I've seen here. Framing the question as How To Prevent Mass Killings and breaking the problem into its parts -- e.g., mental health, gun policy, culture -- is a good approach.
And of course each of those parts is complicated. It will be interesting to see what happens when the broad support for improving the mental health system runs into the question of who pays for it.
The stats on gun ownership and politics are interesting, but gun ownership is a mighty broad category, given the different types of guns (and accessories) and different reasons people own them.