...I very much do not understand your point.

You get that, no matter who provided the gun, the mass shooter shouldn't have done that, right? Even if they thought the gun was only going to fire blanks, they shouldn't point it at people and repeatedly fire. It's only manslaughter if they stop at one death, and manslaughter still carries a sentence.

You get that the DM is supposed to cause evil, right? They create monsters and villains and the players have to overcome the evil in the world. The DM isn't evil because they sent an army of orcs to attack a village, no matter how many villagers die in the assault.

You get that the people in the game aren't real, right? The DM made them up. Nobody is actually dying, no matter what happens in the game. The morality of the people at the table is not rigidly tied to the morality of the characters they play as.

Just so I know where I'm standing here.