The only “someone” in the crowd who would have had a chance at taking him out from more than a football field of height x more than a football field of distance would be a Seal or Ranger with a night vision scoped long gun, a tripod, or VERY steady arms.
Hard enough shot without being under fire and surrounded by a panicked crowd.
500 yards with a 6.5 creedmoor is very easy, assuming you weren’t ducking behind cover. You make it sound like you need years of spec ops training for that. I can show someone how to do that in an hour.