we eat meat because we can

Truly the greatest reason anyone has had to do anything.

and well, also should.

Because we evolved to do it? Are you saying we should do every single thing we evolved to do? Let me just go shit in the woods and die in childbirth then I guess

If modern society falls I'd like to see the vegans after about a year

Gee fucking whiz, maybe the morality of an action depends on context and the available choices. Maybe doing unpleasant things to survive in an apocalypse is actually completely reasonable. Or maybe, just maybe, killing someone to save your own life is still bad.

If killing animals for food is evil then all cats, lions, dogs, and generally all predators are evil.

Evil is a choice. The sun isn't evil for giving people skin cancer. Volcanoes and hurricanes aren't evil. And neither are nonsapient animals that can't just choose to eat meat. If you want to be treated as the equal of a cat then go shit in sand and live off mice. Or, use your big brain to actually do something good.

If you want to realistically improve the lives of farm animals, then go for laws on better treatment while they live.

Yeah, sure. Let's start with a few simple policies to help out the animals: no milking, no forced pregnancy, no genetic engineering to make them lay eggs every day, and no killing. That should help the animals out.