I would venture to say that people commit crimes with guns largely because they can do so from a distance. Guns tend to neutralize any counter attack by civilians by virtue of their deadly nature and how easy they are to fire off with relatively little physical effort. People don't commit knife sprees nearly as often because then there is a greater chance they'll have to defend themselves in a physical confrontation with targeted victims.
Another perspective - when people commit rampages, a good chunk of them are looking to kill themselves at the end of it (and have done so in the past). The advantage with the gun is that it can be instant death. I almost guarantee criminals on a rampage would be less likely to slit their own throat or gut themselves because death is likely not as sudden and is far more painful. Rampagers use the gun as an easy way out of the situation without getting captured.
Also, most people looking to hurt someone are not going to develop a bomb to do it. If it isn't a gun, it's a blunt-force object. The fact that this Dallas guy had a grenade though, that's scary ####.
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