I once stumpled upon an old human ranger and, as always, asked for informations about trouble. One of his tasks involved the hunting of a giant alligator which, according to his words, killed his mother. Upon further investigation by asking about his family he mentioned that not only his mother died to "Denttire something something" but two cousins and four (!) of his five brothers.
I asked him to follow me.
Together we travelled the moist jungles and dry deserts, defending against packs of hungry dingos and wild wolves, challenging the thirst and hunger of the long march towards our destination - revenge. Upon entering the lair of the bloodthirsty beast, which single-handedly almost destroyed a whole family, it attacked us. The meager copper bolts of my companion couldn't pierce the thick scales of this predator and I had to jump into the way of Denttire to prevent another dead family member...
...and in an act of pure kindness I hacked at the legs, damaging the nerves and bones, to bring this beast down and shattering his teeth to render him harmless...
...and then I stepped back to give space to the old chap who managed to journey this far and who was eager to use his crossbow as a hammer of pure justice. A hammer that came crushing down upon the scaly head of this wicked creature. Again and again and again, until Denttire took his last breath.
It was kinda satisfying and I was glad that it wasn't me who did the killing blow as it wouldn't have been fitting.
Letting someone other kill random enemies - the purest form of kindness in DF.