Have you ever studied the ethics of eating animal meat? It's actually an interesting topic that raises a number of questions.
Firstly, there doesn't appear to be anything wrong with eating meat, independent of how the meat was acquired. Say that you find a cow (or some other animal) that has already died of natural causes. Most of us don't find anything morally wrong with eating that dead animal. (We may hold reservations due to sanitation-type concerns, but that's another matter.)
The real moral dilemma relates specifically to "killing an animal for the purposes of consuming its flesh". The GP poster was right-on in his comparison of animals to people with severe mental disabilities. This point represents THE most compelling argument (that I have heard) against killing animals to eat them. I'll try to explain what that argument is.
There surely exist people in the world who are so severely mentally disabled that they hold no more capacity for abilities related to reasoning, perception, and learning than would some of the more intelligent animals (e.g. chimps or elephants). Assuming that one does not think it morally acceptable to kill and consume these severely mentally disabled people, the question is "what makes these severely mentally disabled people different from animals in that it is okay to kill and consume the animals, but not kill and consume these severely mentally disabled people?"
In answering the question, some people will have a tendency to go down the path of "humans are just different than animals...they're special." We may then inquire as to what it is that makes humans special. If the response is something along the lines of "humans have increased mental capabilities", we will likely point out that the original question specifically referred to humans with severe mental disabilities, and these humans cannot therefore be understood as distinct from animals in that regard.
Someone may attempt to maintain, on religious grounds, that "humans have souls and animals do not; that is what makes humans special". If that is your line of reasoning, you may stop reading here and return when you have compelling empirical evidence that supports humans having souls while animals do not.
There may still be those who will maintain that "humans are just special, period" while unable to provide any reason for this assertion. This kind of reasoning (or "non-reasoning") is very similar to thought processes used in attempts to justify discrimination against individuals of a certain race, religion, age, etc. But in our case, instead of racism, perhaps it would be called "speciesism". The definition of "speciesism" would be something along the lines of "discriminating against a member of a species for no reason other than the fact that they belong to the species". Like racism, there is no rational basis for "speciesism", so we will not adopt speciesism-based ideas for the purposes of a rational conversation. (Note that, just like with race, there are times when it is rational to discriminate between different species. I am referring specifically to cases of unjustified discrimination -- making a distinction between two things without reason.)
So where has this taken us? It isn't clear that there is a relevant difference between severely mentally disabled people and animals such that it's morally okay to kill and eat one, but not the other. There are really 3 options here:
1. Determine what the morally significant difference is between an animal and a severely mentally disabled person such that it is acceptable to kill and eat one but not the other.
2. Hold the belief that killing to eat either severely mentally disabled people or animals is morally wrong.
3. Hold the belief that killing to eat either severely mentally disabled people or animals is morally acceptable.