Do you plan a massacre of innocent strangers into your Plan C planning for a drug deal or assassination? No. It just doesn’t make sense. Yes, a massacre helps people take their focus off a teeny little crime somewhere. Like a nuclear bomb gets the cops forgetting about your jaywalking.
It makes no sense to you. But there was shooting at more than this concert. The confusion created my a mass murder is more than enough chaos for a hit squad to use for their escape. the ‘hamdlers’ on scene at the Boston bombing melted into the masses.