This is a good question, but it isn't one that has a satisfying answer. People always have a choice, and in this case, the sniper's other choice was to not kill the man on the opposite roof. He could have just stayed hunkered down behind the parapet until he could make an escape.
Beyond this, there really weren't any other good options, at least that I can see, and all the ones that are left stink:
- The distance was too great, and his weapon too weak, to attempt to injure the other man without killing him. At any rate, that defeats the purpose of being a sniper.
- He could have attempted to identify him by calling out, but during a war that's very likely to get you killed in return.
- He could have stood up an allowed himself to be killed, but that wouldn't have served a purpose.
You have to remember that he was on the roof with only one goal in mind: to kill the enemies of his political cause. To do anything other than kill him would have been out of character. The title of the story tells it all, and in that way, I suppose there really was no other option for him.