salty, doesn't matter how you phrase it - there are people out there that don't believe that any animal should be killed (not understanding that they wouldn't be here if that was the case).
I haven't spearfished, but recently bought a Hawaiian sling for floundering. I had a dive on the weekend and with the amount of large sweep swimming around me I'm keen next time to try my luck with the sling.
Coincidentally (and a bit off-topic), a few years ago I accidentally hooked a gannet whilst trolling for tuna - put up a great fight & although I ended up with sore fingers from the pecking - I managed to safely release it...in saying that, I started to wonder about crossing sports & catching techniques. Ie; using a fishing rod/reel to catch a fox, would be an interesting fight on light gear, although not sure what to do once you land the fox?!

Spear-rabbitting anyone?