Basic vegan argument:
Premise 1: Humans do not need to consume animal products to survive.
Premise 2: Needless killing and torture is inhumane, cruel, and immoral.
Conclusion: Therefor, animal product consumption is cruel and immoral.
Or, it’s explained this way by Humane Meat: A Contradiction In Terms:
“Would you be willing to cut an animal’s throat?”"…And how ethical is it to pay someone to do things that are wholly unnecessary and too atrocious to watch?”
“We have no nutritional need for meat, eggs or milk. Eating meat means, quite literally, eating a corpse. It means robbing the animal of her or his life, and then devouring the body. Animals are all made of flesh, blood and bone, just like we are; they have the same five physiological senses of touch, smell, sight, hearing and taste. They are more like us than they are different.”
There is no valid reason not to go vegan. There are only excuses.