Ummm... I'd would've killed the fish too. Sorry.
I do think it sucks that this happened to be an endangered animal, but if I where down there, that thing would be fillets. Yes, that diver was in the fishes territory, and yes it appeared he wouldn't be killed by the fish... but he could have been. The fish was obviously not shy around the diver and was grabbing at him. Ok, so he only hurt his hand a bit, but what if the fish twisted when it bit and broke his arm? What if instead of the hand the fish went for the regulator or a hose? Dude would be toast!! Yes the fish hadn't done that yet, but the potential for that to happen was very, very real. And night diving is already one of the most dangerous jobs out there without the wildlife!!
Let's say that you are working on a cabin or camping out in the woods, and a bear attacks you (I use bear because I'm from Alaska, please insert any predator you wish) you wouldn't shoot the bear???? As you say with grouper, a bear rarely attacks and is often curious and, dude, you just got attacked by a bear!! If you don't kill it the state troopers will (and yes, most bear attacks are nonfatal, just like most grouper attacks).
Also, I don't believe most people are going to look in their animal id book to check and see if the creature is on the protected species list or not. In the divers defense, there is a very good chance they didn't know it was a protected species. Just because a guy goes diving doesn't mean he's a marine biologist.
Also, I don't beleive that the diver was pointlessly hacking into the fish. He was obviously trying to kill the fish as quickly as possible by slashing its throat. Further proof he wasn't a biologist (or even a avid fisherman). The quickest way to kill a large fish (aside from shooting it, which we do in Alaska) is to cut its gills which quickly bleeds the fish to death (while providing beautifully clean fillets).
I also don't think that the other grouper was trying to help the dying fish. I often would have other salmon following the hooked salmon to the surface. Or sharks swimming around a hooked shark. I think he just wanted to see what was going on, and with that blood in the water, probably try to get a meal.
Sorry to disagree with you, but I just think it is silly to defend a fish when a human life is even possilby at stake.