The Morality
Whether you like the taste of meat or it's something your tradition dictates, I'd invite you to suspend your bias when considering the morality of eating animals.
Most people are against animal cruelty and we all draw a line at a different point in regards to what we consider cruel. The point at where many of us draw that line however is not logically consistent if one believes in human rights as animal rights are the logical extension of human rights.
There is NO TRAIT within animals that if it were not present in humans, would justify us treating humans the way we treat animals.
"Charles Darwin recognized that animals were not qualitatively different from humans and possessed many of the characteristics that were once thought to be uniquely human–but he continued to eat them. Jeremy Bentham argued that animals had morally significant interests because they could suffer, but he also continued to eat them. Old habits die hard, but that does not mean they are morally justified. "
As humans have more consciousness when it comes to the understanding of morality than that of non-human animals, it would seem incumbent upon us to hold ourselves to a higher standard than other non-human animals.
You would not hold a 3-year-old as accountable for doing an immoral action as you would a fully conscious adult as they do not have the awareness to grasp what they are doing. This is why in law; a punishment is significantly reduced if the party is insane or mentally deficient.
With greater consciousness comes greater responsibility.
The Practical Arguments
If I claim to love someone, feed them, treat them well every day only to slaughter them in their sleep, would it not be the utmost abuse of trust and still slaughter?
Furthermore don't animals have the rights to life, liberty and the pursuit of their own happiness outside of satiating our glutinous desires?
There are many questions that come up when dealing with the sustainability of a society adopting a plant based diet. How do we deal with overpopulation?
Wouldn't these animals become extinct if we did not raise them for food?
Would these animals if not humanely slaughtered suffer horrible deaths at the hands of another beast when out in the wild?
Are we not in a social contract with these animals and provided with their flesh as reward for taking care of them?
The answers to these questions however are immaterial and even if there were no answers to them, which there are, they are not moral justifications and would all fall short if applied to human beings.
The numbers of animals that would exist without human interaction and domestication are irrelevant.
Someone not existing without your doing does not give you the right to take their life. By that standard, all mothers and fathers could take the life of their children.
No animal wants to die (evident by their deafening screams within a slaughterhouse) and it's evidently a disingenuous claim that one is in a contract with an animal that entitles him/her to it’s life.
It seems convenient that people want to compare themselves to lions, spiders and other carnivorous animals when advocating for meat eating, despite that lions also kill their young, rape each other, smell each other’s asses upon greeting and some spiders kill their sexual partners.
Unlike lions and spiders, we do not require the consumption of meat in order to thrive.
We are behaviorally omnivores however physiologically herbivores.
Humane Slaughter
Many of us like to appeal to the concept of humane slaughter but there is something so sinister and underhanded about using the modifier ‘humane’ before the word slaughter. It's a play on words, double speak, an oxymoron akin to gentle genocide or righteous rape. In a similar light, Hitlers propaganda minister Goebbels would ask SS guards not to pull kids by their hair when they were being lined up for the gas chambers. The phrase aims to appease our guilt from the harsh reality that we claim to love animals yet pay for their slaughter. It's the worst kind of slaughter as it not only deprives the victim of its most prized possession, it's life, yet continues the injustice by muddying the waters or our most beautiful gift, our conscience.
The Reality
The fact that there are over 700+ million examples of healthy and thriving Vegetarians and Vegans in the world, among them world class athletes e.g. Carl Lewis, Timothy Shief, Alexy voevoda, Venus Williams and the greatest minds that have ever lived e.g. Albert Einstein, Tesla, Leonardo Da Vinci, Pythagoras, Plato, Benjamin Franklin etc would make it self-evident that humans can obtain optimal performance on a diet of plants.
The oldest living groups of people in the world are the vegetarian 7th Day Adventists (Many of which are Vegan) in South California. Another example is the Japanese Okinawans that have a 97% plant-based diet however in recent times have being returning to a modern diet (More meat) and thus seeing an increase in mortality.
Richard Dawkins, Sam Harris, and any ostensibly enlightened thinker of our time concede to the immorality of Carnism because it requires creating double standards or having to accept absurdities like it being acceptable for a more advanced alien race to enslave humans the way we enslave non-human animals to maintain the logical consistency of one’s position.
In the same way that we look back in history at some of the barbaric practices within our society i.e slavery, public hangings and witch burnings, history has shown us that some habits die hard and the arc of the moral universe is long but always bends towards justice.