Most vegans I've known do it because of the cruel treatment of animals in the feed lots, and because they are completely separated from their food system. If we let all the animals breed and never eat them, they would build up until they destroyed everything(deer, rats, rabbits, groundhogs, etc. all do this). If you let top predators exist to control the numbers, well now you're still killing the animals, just in a much more terrifying way for the prey. Also then our children become prey more often.