This YouTube-video shows that gladiators ate a vegan diet: http://www.youtube.com/watch?v=FOWSc8VvOl4
I don't agree with violence (I recommend this article: http://www.abolitionistapproach.com/a-comment-on-violence/ ). This video shows that for physical strength and endurance, a starch-based diet without animal producs is essential.
However, far more important, animals have a moral value being sentient, and veganism is the only way to recognize this and to not exploit other beings.
"The motivation behind vegan living is the universal spiritual principle of compassion that has been articulated both secularly and through the world’s religious traditions; the difference lies in veganism’s insistence that this compassion be actually practiced. The words of Donald Watson, who created the term “vegan” in 1944, reveal this practical orientation and bear repeating: Veganism denotes a philosophy and way of living which seeks to exclude, as far as is possible and practical, all forms of exploitation of, and cruelty to, animals for food, clothing, or any other purpose; and by extension promotes the development and use of animal-free alternatives for the benefit of humans, animals, and the environment."
Read more here: http://circleofcompassion.org/articles/articles-will/article-lovethatword.html