Vegans are claiming it is wrong to inflict unnecssary suffering and death, and thus we can't justify consuming animals, since we can live and thrive on a vegan diet.
"Even if you believe that you have a soul and animals don’t, and that we ought to prefer the interests of a human in any situation in which we must choose— that is, in any situation of legitimate conflict— that gets us right back to the fact that when you are deciding what to eat tonight , there is no conflict. There is only a choice. If you choose the animal product, you are participating in suffering and death in the absence of any sort of conflict or compulsion. Your only justification is that you enjoy consuming animal products or that it is more convenient for you to do so.
Think about it this way. Imagine that Michael Vick were to say that dog fighting was okay because dogs don’t have souls. Would you buy that? Imagine that Vick says God wants us to fight dogs because they don’t have souls. Would you buy that? You would respond to Vick that God’s creating us in God’s image means that in situations of conflict between human and nonhumans, we ought to protect the human interest over the animal interest. So in the situation in which a person is in a true emergency situation , such as being in a situation where she is starving to death with no plant foods to eat, it would make sense for a religious person to say that God wants her to kill and eat an animal and that she ought to do so. But saying that God wants us to eat animal foods when we are not in that sort of emergency situation is no different from saying that God wants us to fight dogs. If you would find the latter to be objectionable—outrageous perhaps— you should find the former so as well.
Francione, Gary; Charlton, Anna (2013-06-24). Eat Like You Care: An Examination of the Morality of Eating Animals (Kindle Locations 569-570). Exempla Press. Kindle Edition.
There is no ethical justification at all of taking the life of a sentient being: http://articles.philly.com/2009-08-14/news/24986151_1_atlanta-falcons-quarterback-vick-illegal-dog-dog-fights