A grenade would do much more damage. Lob it into the room, close the door and run. But are grenades legal to buy? NO
Yes, one could come up with their own improvised explosives, but they are inherently dangerous to handle and risk going off prematurely. The movie theater killer defaulted to assault weapons because they are convenient. His explosives were all rigged up and waiting for police at his home.
But this is besides the point. The killer wants to see his victims die
. He wants to aim his weapon, pull the trigger, and feel the satisfaction of killing someone. Lobbing a bomb and running away does not deliver the raw feeling of the moment they crave.
The TROUBLE is our society's indulgence in these assault weapons, making it a "past time" to load up and unload on multiple targets in seconds. You learn how to use one then train your kids how to use them too... never mind that one of them may be mentally unstable. Shooting for sport is fine for single action. But for weapons that can kill dozens in seconds? I'm sorry, there's no good reason for it.