A license to kill?

Articles or pictures about meat eating often stimulate big discussions with a lot of responses on the web. Tomasso Ausili is an Italian photographer who won a World Press Photo of the Year Award in 2009, for these pictures (only click if you can stomach it). In the long discussions under the pictures I read in Italian: "we are omnivores and I don't see any reason why we should feel guilty."
Being an omnivore, this argument goes, gives us a license to kill. I think it would, really, if being an omnivore meant that we wouldn't be able to survive without meat. But as it is, being an omnivore just means that in the course of x years of history, we have tended to eat anything, both vegetable and animal products. That's all that it means. Not that we need meat, just that we are used to having it.
But how can being used to have something in itself justify the fact that we continue doing it, when (or if) we feel it's a wrong thing to do?

So often I hear people, also vegetarians, say that we are allowed to kill animals for food, but we should make sure that they get a good life. First of all, hardly any omnivore who thinks like this, acts on it (acting on it would mean eating only meat from not just merely organic but specialised small scale "personal" farming, where the animal is treated better still than in "conventional organic" farming. Secondly, where did we ever get this idea that there's nothing wrong with killing? I have never read a good explanation of why it would be wrong to kill a human, but ok to kill an animal. It's hard to construct an argument for this that makes sense (saying that humans are humans and animals are animals is not an argument, or at least not a morally relevant one). If you want to see what's wrong with killing in just one image, this photo from the previously mentioned photographer says it all, in my view (and I just read in Every Twelve Seconds that the guy doing this job - he's called the "knocker" - needs to visit a psychiatrist every three months. It's in his job description.)

Killing is finishing a life. A life that wants to live. We don't need to eat meat. There is no license to kill.