The very premise itself: there’s a sniper at a football game, armed to the teeth with a custom sniper rifle. He’s slipped through security and perched on the scoreboard. He has no qualms about using said rifle. And we never find out WHY he did this...
The stampede and carnage caused by 91,000 panicked spectators as they flee the game when the sniper starts firing.
The sniper coldly executing a bicyclist at the film’s start for target practice.