If you can't eat it, don't shoot it. I have left huge groupers, bigger than me, alive in mozambique. I would not know what to do with that much fish.
That said, if I saw a marlin/tuna or sailfish I would go for the kill without hesitation. Hmmmmm....
While I did shoot huge groupers (AKA jewfish/Goliath Grouper) as a teenager in Florida, I wouldn't do it now. But then I wouldn't shoot a marlin or sailfish either. They aren't very good to eat, and I used to release a lot of marlin during my rod and reel years. It would seem inconsistent to kill one now. I've passed up two easy shots on marlin.
But then I'll confess that this is an emotional decision. A marlin has no more right to live than a flounder, but then I think this entire discussion is more about emotion than science or logic.