If we eat the flesh of the animals by killing them, shedding the blood, giving much pain to the animal gods, we got to pay for it. There are so many reasons why we should never kill animals. Killing animals is very very heinous karma.
Animals are always happy and serene, You can never see a 'sad' animal. Every member of the animal kingdom knows that death is an inevitable phenomenon. Every animal-being automatically understands not to expect anything from other beings. Every animal-being perfectly understands that everything happens according to nature's laws.
Human evolution is so designed that, in the first phase, we develop a strong separate-hood, a strong ego-hood. And then, in the second phase, we have to make equal efforts to erase the separateness, erase the ego. These are the two imperative phases in the evolution of the human species. In these two phases, the humans differ vastly from animals.