If you don't think that the SS is playing by different rules, then you're sadly mistaken.
An unauthorized kid crawling on a rooftop 150 yards from the former president with a rifle is more than enough justification to take the shot. Welcome to the United States, where you don't have to watch someone get shot in the head before taking action.
For instance. Minnesota law allows a person to use deadly force if the individual reasonably believes that the person or another person is at risk of great bodily harm or death. Now you have a duty to retreat, and it has to be the only viable option as well. In this case, there's no logical reason the kid would be up there with a rifle if it wasn't to attempt an assassination on Trump.