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The 911-Call from Sandy Hook where a School-Nurse called for help. In the Background you can hear Gunshots and Screaming.

The moment when Anton Lundin Pettersson took that picture with the two girls thinking it was a Halloween prank after he had already killed 2 people. They probably had the biggest "Holy Shit" moment after they heard the news about the stabbing spree.
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Another one would be Robert Hawkins getting employees to open up the safe in the guise that he'll let them go if he got the valuables and cash inside but just gunning them down like target practice a few moments later.

Ali David Sonboly hacking into a Facebook account and leading his victims to their doom by baiting them with a free giveaway at McDonald's as well as asking one of them if their burger tasted good just a few seconds before the massacre started

Maurice Pahler reportedly having a dead-eyed yet accomplished look on his face as he was dragged from his car after running down 150+ victims.

Gabe Parker checking if anyone from his band class was around just before the shooting and blending in with fellow students afterwards.

The time it took for Friedrich Leibacher to commit suicide. He didn't hesitate at all. It only took about one and a half seconds for him to shoot himself after dropping his rifle.