The rooftop was covered from the SS sniper position. The relevant question is how much time was the sniper aware of the assassin on the roof top, and what were his rules of engagement? The answer to these will help determine if this was incompetence or designed failure.
The raid on Mar-a-Lago had more aggressive rules of engagement.
The othe question is if the police officer had a good shot to make. He may not be able to have a good shot at the guy. Everyone assumes thr officer had a good view, aim, and can make a kill immediately. It’s not like the movies. We don’t know what the officer sees.
One. You don’t want to miss and cause a panic.
Two. If the police officer misses, then the assassin just starts firing killing people.