It's within the kill and accuracy range of the gun, so ya I'm going to take the shot and I'd do it again. The only reason I did get a kill shot was because of poor aim from this inexperienced shooter. I did nail two hogs the same day from about 15' away with perfect head shots.
I do feel bad that I wounded it, but it's not much different from all the lobsters you see with missing antenna's. The intention is obviously to catch/kill, but hunting isn't always pretty (in the water or out).
Shooting from 20' away on scuba is not sport and as you said your poor aim from an inexperienced shooter has left a fish to suffer needlessly, hunting is shooting and catching what you are going to eat either in or out of the water. If you shoot it and wound it normal practice is to follow through that is the sport, not wildly shooting everything in sight from maximum range and then looking for the next target. Get a grip.