Can we please stop pretending "Guns don't kill people, people kill people" is anything but the most specious of oversimplifactions?

Yes, a person killed all these people. But he wouldn't have been able to if he didn't have easy access to firearms and a large cache of ammunition. Everything else that can be said on the subject of what would or would not have prevented this is speculation that is, at best, contingent on a large number of factors about which none of us can possibly know enough to make anything but the most tentative of suggestions.

But it's absolutely a cut and dry truth that it would not have been possible for him to do this if he (he specifically) had not had access to the kinds of weapons necessary to enable someone to commit these kinds of murders.