It's amazing that this argument is still going on after all these years; Morrissey did not come up with the abbatoir=concentration camp analogy, nor did PETA.
Isaac Bashevis Singer, a Polish Jew who fled his country a few years before Hitler invaded and who lost family in the Nazi death camps is the man who penned the phrase: "in relation to animals, all men are Nazis; for the animals, it is an Eternal Treblinka." Singer was a Nobel prize-winning author, a humanitarian and a supporter of the notion of animals rights. He wasn't some armchair bleeding-heart, he watched the madness that gave rise to the concentration camps unfold, and he had no problem equating the industrial killing of animals with the horrors of man's inhumanity to man.
It doesn't matter if a chicken or a cow knows what a Nazi is, it matters that animals feel pain, that they suffer, that they are helpless and that they are killed in mind-numbing numbers with no compassion whatsoever. The Nazis dehumanized their victims; they were killing "vermin" and not men, women and children. We devalue animals to the extent that they are now "harvested"; the same desensitization is required to kill on an industrial scale. The lack of compassion and empathy is what allowed the Nazis to slaughter their victims, and it is the same lack of compassion and empathy that allows us to slaughter millions upon millions of animals every single day. We are not talking about moral equivalence, we are talking about the human capacity for disengagement with other living things, about notions of human mercy, justice and the right to petition a loving god. It is a profound philosophical point.
As for "Meat is Murder" being literally true, it is about as factual as "Silence=Death" another phrase from the period that changed minds and saved lives. Morrissey should be very proud of his song, and its continuing emotional resonance.