All I'm trying to say is that for the average person a gun is the most destructive thing available. A person who is emotionally unbalanced can reach for a gun and do the deed. Whereas if they have to manufacture an explosive you're looking at some time, time to stop and think. Plus pulling it off is tricky too, you have to manufacture or obtain a detonator, you need a timer, etc.
I understand your point, and agree, in part. For the average person, guns are the most destructive thing easily available. Of course, gasoline and matches are pretty easily available too....
But it isn't the average person that is committing the mass killings.
Look at some news from Asia. Guns are not common, but knives, swords and machetes are. There are many, many reports over the years of mass murders (sometimes over two dozen) commited by sword or machete wielding berserkers. The killer of all those children at Sandy Hook could have done it with a machete. Do you think the end result would have been significantly different if the killer had to run a little bit to catch fleeing victims? I don't.