I don't think the Geneva Convention requires you to kill someone in the name of mercy.
I too have never heard that. But generally Snipers shoot to kill, one shot, one kill …
But it was not unheard of in many conflicts that a Sniper or any other Grunt accidently or purposely wounded an enemy. And then use the WIA as "bait". E.g. you saw this in the movie, Full Metal Jacket. As we study the "art and & science" of warfare. We see it can be a pretty violent and brutal enterprise.
Does this fall under 'war is hell'?
Yes, very much so … from what I can tell.
What are the legal and moral guidelines on this subject?
Follow the GCs and ROEs … However "moral guidelines" = survival regardless, of both you and your comrades/troops under your command …
What did Patton say about "killing the other poor … etc."… ?
I really don't have much compassion for many "enemy combatants" AKA terrorists.
Very much so. I'm sure we saw this occur when dealing with the plethora of jihadis terrorists, etc. after 9/11. Or even in Somalia in '93., etc.
Frankly if I was still serving, enemies like jihadis e.g. AQ, Taliban, ISIS, etc. would have a very low survival rate. In the units I would have lead/commanded. We always have to be aware of the GCs, ROEs, etc., of course.
But when in doubt I'd tell my troops to shoot to kill, don't take any chances. For better or worse, events like Blackhawk Down, 9/11, Taliban, ISIS, etc., brutalities, crimes against humanity, etc., Would only make "terminating with extreme prejudice" a standard. If they are still shooting and not clearly surrendering. You have no choice but to kill them. In large numbers and often if possible. Even if you see a Jihadi is wounded and surrendering. When in doubt, wipe him [or her] out.
And as we saw in WWII PTO with the IJFs. Just because someone is surrendering does not mean they actually are. And they want to blow themselves up and take you and/or your comrades with them. Read the book "War Without Mercy" about this in the PTO.
So bottom line IMO, Snipers generally shoot to kill, especially priority targets. E.g. Leaders, Commo, Heavy Weapons, etc. And if you really hate your enemy, that would probably not be too hard to do. E.g. albeit not a Sniper, the SEAL that got UBL in his sights. IIRC double tap head shot.
Again even in both world wars, and on. Hatred of one's enemy is nothing new. We saw this e.g. with the Allies vs. IJFs and Russians vs. the Nazis, etc. in WWII.
Killing the enemy is why you are there. And killing them in combat helps insure not only your own survival but more importantly your comrades. "Dead men [or women] tell no tales." And they can't shoot you either.