Because killing animals is always wrong. Therefore, I believe that killing an animal is morally wrong even if done in self-defense or defense of others.
Because killing animals, even if done painlessly, is wrong absent exigent circumstances and killing animals for food, clothing, cosmetics, etc., does not amount to an exigent circumstance.
Because although it's unclear whether killing animals painlessly is wrong, animals that are killed or used for food, clothing, cosmetics, etc., are usually treated in an unjustifiably cruel manner
Because participating in meat consumption is extremely harmful to the environment
Veganism is not morally appealing.