I mean, its a move.

and to do em all at the same time is to prevent them from organizing a reaction/defense/counter. its also kind of scary if maybe you were in the margins, sympathizing with one of them quietly, and now they're all dead. see also Operation Hummingbird.

or, a fictional but taken to the extreme example, when Walter had those biker freaks do all those people in prison at the same exact time.

the message is clear: I can do whatever because I have enough killers to orchestrate some shit like this.