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.

Australia had neither a War of Independence, nor a Civil War.

Fun Fact: Most Australian's don't know the name of the first Prime Minister because it was so uneventful. A bunch of people sat down, signed some papers, and Australia was founded as a country.

Lots of countries were founded by violence. Europe, for example, was convulsed by violence for most of the past thousand years leading right up to the middle of the last century.

One big difference in the USA is that the right to own guns was somehow inserted into its Constitution among the rights to expression, assembly, fair trial and so on. I can't think of any other countries in which owning a gun is explicitly listed among the set of human rights.

It starts from the foundational document, the Declaration of Independence:

We hold these Truths to be self-evident, that all Men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty, and the pursuit of Happiness

We have rights to life and liberty, and the right to private ownership of weapons acknowledged in the 2nd Amendment is an instrumentality to help ensure that. It should also be noted that the Bill of Rights (1st though 10 Amendments) were a price demanded by the Anti-Federalists for accepting the Constitution.