I may not understand the specific geography of that roof and that ladder, but it seems like the local cop went up the ladder, saw the guy on the roof, the guy pointed a rifle at the LEO and the LEO retreated down the ladder to ... get help?
It seems to me that one smart strategy could have been ... go down 2 steps, out of sight, draw your weapon, then surprise the bad guy by suddenly appearing again and shooting the man who just pointed a rifle at you.
Running away ... they teach that at the Police Academy?
That cop could have done anything, including ducking down and shooting into the air several times to rattle the assassin, alert SS to the assassin, and trigger the removal of the president from the stage, anything that even a cowardly cop could have done would have worked, except of course quietly departing the scene.