If you weren't in heavily settled areas, you might be attacked by Indians, sometimes put up to it by the French or the British. By virtue of being a while male over a certain age, you were in the militia.
Now, the gun-bearing clause doesn't pertain just to the militia. But in order to have a well-equipped and trained militia, you had to have guns fairly common among the population for self defense. Even if the guns then weren't what they are now. However, advancements had been made during the lifetime of the framers of the Constitution, and I don't see any reason why they thought that there'd be no advancements in the technology. So at least the next step or two after 1790 in firearm 'progress' is handily included. Semi-automatics came later, but as semiautomatic pistols guns became more common in the US in the last 70 years the homicide rate has declined; we've had some of the more lethal weapons for a while, but the recent increase doesn't track that well with their ownership rate.
Dueling was accepted by many, so the idea of using lethal force to settle a dispute wasn't unthinkable. But there were rules, and one of them was that if you screwed up you screwed up. However, fighting to defend your honor--which is what many murders these days boil down to--was a accepted practice for some. (No, I have no idea how it fit in with homicide laws.)
However, social attitudes among the educated and converted were different. Rage wasn't authentic; grievance wasn't celebrated. People had a harder time being strangers. Groups were separated by distance. If you nurtured a grievance against a group, it would be a group fairly like your own and you'd go to fight it out with them and resolve it before you butchered their entire family or you'd be stopped by your peers. It was unlikely that you'd be supported in something that foolhardy and find a peer group that would say, "Atta boy!" Unlike now. Or it was against a group that was sufficiently distinct that there'd be inter-group hostilities which would, again, be definitively resolved.
The founders also assumed a different population. They assumed that the population of a viable democracy had to be virtuous, and had a specific meaning of virtue in mind. Many consider their definition to be foolishness, even if JFR still hewed to something very much like it, if not identical with it. Even those that still hold to that definition these days find that they feel like fools taken advantage of when they practice it: It presupposes a commons that everybody but they has set upon to divvy up while they continue to maintain it at personal cost.
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