There is greater survival value in being an omnivore than a vegan. Do vegans imagine cattle and other domesticated livestock chose to be vegan for any reason other than because their physiologies were poorly adapted to being carnivores or omnivores? Even some plants eat meat. They'd probably be even more aggressive about it if they were better adapted for it. Their energy budgets preclude movements quick enough to capture and ingest a more varied diet. Even if they did, animals would quickly adapt to avoid man eating kudzu.