Carnist, I believe, is a term used by vegans to refer to those who support ideologically that it is acceptable to use animals for food, rather than someone who eats meat. There are people who do not eat meat, for religious or cultural reasons, and those are generally referred to a vegans, in the same way there are those who eat meat for cultural reasons (my father did it, his father did it, I've never really thought about it) rather than supporting some ideology. For vegans to label meat eaters (more accurately omnivores) as carnist, their ideological/political opponents is, well, not really offensive, but it does rankle.
As social psychologists say, there is no in-group cohesion without out-group hostility, but it is a shame for vegans (for which I have some sympathy) to isolate themselves in this manner, particularly if they want to change the world for the better.
Visit arid Africa: eating an animal before it was /used properly/ (could be as simple as fertilizing the land here in southern Morocco) is considered wasteful.
I wouldn't know about Africa. But I know a thing or two about keeping animals at the edge of the arctic.
If you butcher when the animals are old, you have quite a bit lower risk of losing the meat due to disease, because old animals fall ill more than young ones.
If you butcher, you have some control over the timing, which helps with using the food. You don't have perfect control, but more than random, and you reduce the risk of having to throw anything away.
So if you use the animals both live and dead, it makes sense to optimise for keeping them alive long and die while still healthy (healthy enough to eat/use, if you want to be cynic).
Does my car stop working because its old? Strictly speaking no, there will be a cause of failure, in practice everything wears out, age has increased the chances of things wearing out, so after a certain point age becomes a 'good enough' reason.
I suppose if you want to be pedantic, a common failure mode isn't actually 'correct'. A heart attack isn't the cause of death, its oxygen starvation, so I'm not sure more 'correct' equals more useful.