I've been thinking about it. And, without being there, and not knowing the actual facts, I can give this officer some benefit of the doubt. Have you ever climbed onto your roof from a ladder using only one hand (if he's supposed to have his weapon drawn)? It's not stairs. He was using both hands to try to lift himself onto the roof, so he wouldn't have ready access to his weapon. The perp supposedly aimed his rifle at the officer.
The officer's head and torso would be exposed and then his legs as he swings them up to get on the roof. If he stands on the rungs and tries to peek over and shoot, he's still exposing his head and possibly torso, trying to maintain balance on a ladder, and trying to possibly get a shot off shooting toward the perp, and from the location of the ladder, shooting toward the perp would also result in shooting toward President Trump and the crowd if he misses a shot.
Again, this is armchair quarterbacking. I wasn't there. There is video of the perp climbing the ladder and crawling across the roof, but I have seen any video of the LEO who climbed up the ladder after the perp.
Again, this is armchair quarterbacking.
Firing warning shots? Problems there too, could have distracted everyone away from the real problem.
This should be "but I haven't seen any video"