Well, if Mr. B were a pacifist and strove to remain as such, then I'd say he acted immorally. If Mr. B was trying to become a kind of warrior, and Mr. G were the same, such that each of them were essentially "In Agreement" on how one treats an enemy, then I'd at least say that what he did was morally permissible. On top of this, if the only reason they were enemies of each other, even with an explicit agreement, was because the two were bored and wanted an enemy to spice up life, then I'd likely still say they acted immorally, as I think there are healthier ways to make life exciting.
But, as I had given an example of in my post on the first page, until Mr. Gravity hit the ground, he was not dead.
One can't say with absolute certainty that there was no way he could have survived that fall, until he actually hit the ground.
What if, instead of the temporary awning that had been installed in my first post (which Mr. G may have seen), something else happened that caused him not to die.
The driver of an open truck filled to the depth of 12 feet with cotton balls could have been driving around the corner, lost.
He didn't know whether to turn left or right at the stop sign, and, taking a chance, turned right, while Mr. G. preoccupied with jumping, and Mr. B, preocupied with shooting, did not notice.
The cotton ball truck could have been passing directly below him, saving his life.
Mr. Bullseye did not act morally, since he could not have been certain Mr. G. would not survive the fall.