That is hilarious because the sniper agrees with you. He says that a true sniper would go for the kill shot and not the head shot. But that a head shot would be more horrific and would inflame people all the more. They didn't just want to kill Trump they wanted to inflame riots so they could institute martial law.
Pride goes before a fall. I think they assumed they could kill him any way they pleased. Also, if you are aimed at the head, take a shot, it takes 1-2 seconds for the bullet to get there, then it takes 3-5 seconds to know if you killed him. Since they are aimed at the head they don't change their aim instantaneously to the neck. So they clipped off another two shots that also missed Trump but hit spectators. Then some other gun let off five shots, not at Trump who was already down and out of sight but just to create panic and a riot. Then there was one more shot fired. Possibly from Crooks, so that was when they shot and killed Crooks.