The most compelling argument that can be made for this proposition comes in two parts. First, we must argue that it is ethically proper to kill and eat animals because they are not as important as human beings. Second, we must argue that human beings nevertheless have a duty to prevent animals from suffering in any ways that are not necessary.
In order to say that we may kill and eat animals, we must assume that animal life is worth less than a human life. While some people might argue that the two types of life are equivalent, it is not hard to argue that animal life is less valuable. It is clear, for example, that lower animals do not have the sorts of emotional and spiritual lives that we people have. At least, if they do have such lives they show no outward signs of it. This would mean that their lives are less rich and, therefore, less valuable than our own. (Of course, this would also imply that the nearer an animal comes to having human emotions and such, the more unethical it would be to kill and eat it.)
If animal life is worth less than human life, why must we refrain from cruelty to animals? One way to think about this is through thinking about the laws of war. We fully accept that people will die in wars, but we have rules that are meant to prevent unnecessary suffering, even in people whom we are consigning to death. The same attitude can apply to animals. Even if we are fully entitled to eat them, it does not mean we are entitled to treat them badly. There is no need to make them suffer and therefore we should not do it simply for our own convenience.
This is a very difficult question to answer. My best answer is that cruelty to animals is unnecessary and does not do us any good while eating animals is useful and to some degree necessary.
I do not believe that animals really have a right to life in the same way that people do. I do not believe that they are sufficiently advanced in terms of intellectual capabilities and I do not believe that they have, for want of a better term, a soul. Therefore I have no qualms about eating animals.
However, it is not necessary to be cruel to them. Cruelty to animals does not benefit us. Instead, it is really just a manifestation of moral failings in the person who does it. There can be no good in abusing something that is completely helpless and in your control.