My understanding is that a horse is killed when its profit no longer matches its expense. Is this something Jews should/may do? I can think of some pros and cons, but wonder what authorities have said, whether from an halachic perspective or otherwise.
This would seem to be comparable to hunting, minus the sport and danger aspect of it. Namely, killing an animal for reasons other than food or self-preservation.
The Nodeh B'yehuda discusses hunting in Mahadura Tinyana Yoreh Deah Siman 10. He writes that there is no prohibition of tza'ar baalei chaim when you are not leaving the animal alive to suffer. He also writes that there is no prohibition of bal tashchis when it is being done for a purpose. (This is similar to the halacha of cutting down a tree which isn't worth its upkeep.)
The issues which he brings up with hunting seemingly wouldn't apply here - this isn't a dangerous involvement, and isn't engendering a sense of violence or cruelty, as you aren't doing it for sport.
It would seem from Chulin 7b that killing an animal for no derived benifit is definitely 'bal tashchis'. And there was even a purpose, to kill the dangerous donkey. And yet, it was called 'bal tashchis'.