Technically, this is not an argument for vegetarianism. Rather, it's an argument against imposing pain and suffering on animals. If an animal doesn't suffer and is killed painlessly, nothing in this argument implies that it's wrong to eat it. The same goes for an animal that has died of natural causes. But it is an argument against the way that most of our meat is produced. And it does entail that most of us are living deeply immoral lives.
It seems to be an interesting human behavior that we are so easily able to ignore the pain and suffering we contribute to if it is not in our faces. I know people in my own family that have made brief efforts at becoming vegetarians but failed after a week. In these particular cases they made the decision to stop eating meat based upon a desire to be healthier as well as exposure to media that had, for a moment, alerted them to the suffering of animals in the factory farm system. But one week into the process the images of suffering had faded.
Of course this can be applied in a general way to the activities of our daily lives. Much that we do and consume is built upon a foundation of suffering that we do not see. We also play word games with ourselves. It's easier to skip over the details if we just think that the "goods" we purchase are the result of legitimate and sanctioned business exchanges. If we were to dive into the details of the larger context, the details of global capitalism, we might well decide that we don't like what's happening. We might decide that we're not purchasing "goods" at all, rather we are purchasing objects that represent a certain amount of pain and suffering.
Are we cruel? I think the answer is yes but perhaps the more important question is why are we able to so easily lie to ourselves.