It is only “not possible” in a vacuum. It is entirely possible in a world where wealth and poverty coexist.

Humans come in many different sizes. Before guns, people who were unusually strong could bully everyone else. Now, however, that is not possible. Civilization was, to a considerable extent, made possible by effective weapons for weak people, including guns, which made it possible for almost anyone to defend themselves. You refer to “mutually assured destruction” as though that does not, in fact, bring safety, but of course it does bring safety — without it, violence works as a strategy.

I do not think it is a coincidence that in the ancient world, looting was considered a more effective strategy for wealth acquisition than making things. “Conquerors”, or more properly, looters, were celebrated in song and story, because they had the most stuff. Carpenters and potters were poor fools — big men who could beat them up and could take their stuff were the ones who lived good lives, not the makers. If you were young and strong, becoming a “soldier”, or more properly at that time, a looting bully, was probably a good professional choice. Read any ancient account — the Anabasis of Xenophon is a pretty good one to start with.

However, now that lifestyle is entirely impossible. You cannot succeed with this life strategy any more. “Mutually assured destruction” is not at all a bad thing. It shifts what a successful evolutionary strategy is in a very good direction, towards one where the carpenter and not the looter ends up rich.

Consider, for example, the case of a modern armed robber. He has to survive encounter after encounter with victims. Victims, on the other hand, will see an armed robber only rarely. If victims are never armed, the robber can continue his profession indefinitely. If, on the other hand, victims are almost always armed, it is impossible to pursue armed robbery as a profession. Even if the robber is substantially better in a fight, and wins the shootout in 75% of his encounters, he won’t (on average) last a week at his job.

This is, in fact, an example of precisely why you’ve inverted the conditions. This isn’t a “tragedy of the selfish”. In fact, if everyone is armed, often those who are armed might end up sacrificing themselves to stop malefactors. It is individually far more dangerous to be armed because if you simply surrender you are more likely to survive an armed encounter — but the malefactors will not be able to thrive in such an environment because to succeed with their strategy, since they have to survive multiple encounters.

I do not think it is a coincidence, by the way, that violence has dropped over the centuries since the invention of the gun. I think that those more inclined towards violence are evolving out of the gene pool.