Not trolling here… but why do they have to be killed?
I really think this is moot and quite a pointless discussion. Horses can have their legs fixed, they are put down because its costly, although some rich owners do pay to fix them, others don’t.
Some breakages can be fixed, yes, but others not. Plus its not always a broken bone, the tendons are a common thing to snap/tear aswell.
One of ours half tore a tendon on its front hoof. 8 weeks of box rest, gentle walking out every day in hand rain hail of shine, injections etc etc etc … no cost spared. After the 8 weeks, it went back outside. 3 days later it can in hobbling … had torn it again cause it was running around like a hooligan ( he was like that ). Was put down the following morning. Somethings just cant be fixed and its simply crueller to keep them going.
Generally, the front legs take about 2/3 of the weight, so any weakness in the front will be shown up quickly.
Looking back, I’ve always said that if we were faced with the same situation, I would have the horse put down at the start ( if the prognosis was not good ). Its not fair to keep them in the stable, and whilst mine behaved, I didn’t half feel sorry for him every time I walked away and he just stood there , head down, depressed. In such cases, its about quality of life, and not preservation at life at the expense of quality of life.