This seems like as good a time as any to bring up something that bugs me about Rimworld, and that's dealing with the aftermath of an attack.
Let's say you just drove off a raid. You have several dead raiders, and maybe a couple of them with relatively minor wounds that can be treated. But then you have one or two raiders that are clearly on their way out. Massive bleeding, damaged vital organs, missing chunks of torso - whatever the reason, these guys will probably die before they can get medical help, and even if they don't there's a better than even chance they'll die in bed. Some might survive treatment, but be so entirely damaged that they'll never move around on their own short of a bionic spine or some such thing.
What is the best response? If you take them prisoner and they die, your colonists' moods will suffer because you let them die in your care (even if you did everything humanly possible). If you mercy kill them, moods are still negatively affected. It isn't quite as bad, but it's still a penalty when all you're doing is giving a person that intended to kill you a clean and relatively painless end.
No, the best solution (from a purely gameplay perspective) is...to leave the wounded raiders writing on the ground, possibly in the freezing cold or blazing sun, until they die in agony. Your colonists will not suffer a single negative moodlet from this, other than the minor one from witnessing an unburied corpse. Heck, you can (and in survival situations, should!) strip them of their clothing before they finally stop breathing so you can use it for your colony.
It's a really weird disconnect for me. I'm not saying everyone in the colony should suffer mood penalties from not tending to wounded attackers (maybe just the ones with Bleeding Heart traits) because, after all, they did just attack you. You might be too busy saving your own people to deal with surviving raiders. But mercy killing really shouldn't be penalized. It turns what could conceivably be a good act into one that your own colonists will despise. But standing around and watching someone with three bullets in their gut squirm in pain for hours is somehow totally okay.