How about this: Neither are to blame.
Other countries with comparable gun ownership have far, far lower rates of gun violence. Even crazy third-world dystopias are less violent.
The problem is cultural. American culture, obsessed with a Manichean view of the world in which everything is either utterly good or utterly deserving of destruction, is the reason why tragedies like this continue to happen. Guns are just the way they choose to make it so.