Sorry - that's a pretty silly argument. Countries that formed through some form of peaceful negotiated settlement exist, but most formed either through conquest, revolution, or through an imposed partition. I don't have numbers on this, but running through examples in my head, especially for the developed countries I'm familiar with, I can only think of a handful.

I'm not sure what you have in mind when you say "founded by violence"...

Australia was at least as wicked with their aboriginal populations - and has a vastly lower level of gun violence.

The Netherlands was birthed in a revolution lasting 8 decades, and also has negligible gun violence.