Pardon the late reply, but I would say the major thing here is that the gun requires minimal mental control over your body to execute a murder successfully. There's no time to hesitate once the decision to kill (=pull the trigger) has been made. All it takes is for the gun to be pointed in the right direction and a quick twitch of finger. With a machete, knife or an axe, one has to execute full physical assault. That takes concentration and effort and there are many more variables that can go wrong. Dynamite? Kind of, but still requires dedication to carry out the murder as long as the fuse is burning. So that's the thing with guns in my opinion. All it takes is one signal from the brain, a twitch of finger and the target has a bullet in them. The murderer might regret the decision immediately afterwards, but with guns the result is already final.