Well, as you obviously know, it's impossible to travel back in time, remove the gun, and see what happens.
We are left with logic and common sense:
For example, what if the guy you wanted to murder wasn't standing with his back to you? Attacking him with a knife, especially without training would be a high risk proposition. He could wrestle the knife from you, and kill u with it.
If both have guns, there's another game theory layer, the only option that will guarantee your survival is to shoot first. You don't even need to know for sure that he has a gun, just the reasonable chance is enough to make it advantageous for you to pull the trigger.
That's just two pretty common situations where guns raise the stakes and makes murder much more likely.
How hard is to understand that letting people, PEOPLE, the average and below idiots, carry and play with tools which can murder anyone on the slightest temporary mental whim. It's insanity.
For collectors, gunlovers, and people generally able and motivated enough to take a course, do an exam and get a mental checkup, let them have whatever they want I say.