If both my hands were busy trying to pull myself up over a roof edge (where I could not draw a side arm) and someone pointed a rifle at me, it would not be cowardice to duck back below the ledge. It would be survival instinct.
There are a few times when survival instinct can be over come, and usually only with stress training to inoculate oneself from the natural survival instincts. Rushing into fire to save the life of a loved one falls into that category. Given the roof shooter had not fired yet, and the cop didnt know what to expect, and it wasnt yet clear that lives were in danger, I am going to give the cop some leniency here, for the moment anyway. These cops were not trained SEALS, and may not have even been SWAT. Street cops dont get nearly as much training as you might think.
Circumstances obviously changed very quickly after the initial encounter by the cop.
Just because he wasn't a hero doesn't make him a coward in this instance. I need to know more, were they properly trained by the SS as to what their duties were, why wasn't someone posted around the building and did they call "gun" on the radio instantaneously.