The Secret Service, by design, left open the best location to whack Trump. The would-be assassin had no right to think that that location would be unguarded, unless he was told so in advance. Granting the assassin the best location to whack Trump, the Secret Service grabbed the best location to whack the assassin. This is similar to JFK's assassination role for Jack Ruby killing Lee Harvey Oswald.
While the Secret Service people were watching the would be assassin with the gun, which they did for some time, do you think they could call over to him "Hey, you with the rifle, what are you doing there?" Of course not.
The Secret Service men on the roof were conspicuous accomplices in the attempted assassination of Trump. Do you think this story will play out this way? Fat chance.