The standard legal form of slaughter for animals is for them to be "stunned" and then have their throats slit. For chickens and pigs, the stunning is generally done with an electric shock, and for other animals a pneumatic bolt pistol projects a metal rod into their forehead. It is claimed that this renders the animal 100% unconscious, but if you actually look into the facts, slaughterhouse workers will admit that there is no way to verify that this is the case for every single animal, and indeed the process doesn't always work, isn't followed routinely, and indeed, the "stun" can wear off while the animal is being killed.
But regardless of the fashion of execution, there isn't a justification for taking the life. It is still taking the life of a sentient being, for your enjoyment ultimately. If somebody killed your companion animal, I doubt you'd say "that's fine because you did it humanely" as described above.
- Vegan Sidekick
The words 'humane' and 'slaughter' put together, are what is known in the English language as an oxymoron, i.e. 2 words that contradict each other when put together. To use the term 'humane slaughter' is as nonsensical as to say 'humane rape', 'humane slavery', or 'humane holocaust'—regarding the latter point, some synonyms for 'slaughter' in the dictionary are 'bloodbath', 'massacre', and 'holocaust'... given that it does not make sense to use the term humane for any of those 3 words, neither can it make sense to say it for the word those synonyms derive from.
Ask yourself this question: is there a nice way to kill someone who doesn't want to die?
Given that animals want to live, and value their lives as we value ours, there is no nice way to kill them.
In any case, anyone looking at the methods we use to kill farmed animals can see for themselves that it's not 'humane'. Whether the animal is stunned with a bolt gun or prongs, or whether it's by gas chamber, or whether they are killed via the Halal/Schechita method, these are not exactly methods we would use to euthanise even someone who did want to die.
And humans evolved as omnivores. Without meat, or brains would not have evolved as large and we would not be as intelligent.
I'm not saying this doesn't mean we could live without meat. As a matter of fact, the world would improve if only people didn't eat as much meat and get fat, but humans still need meat.
Wild animals kill to survive. They must kill to eat, otherwise they would die. Whether they kill on instinct or are aware of their predicament is irrelevant, we are not in their situation. If you live in modern society and have access to crops, vegetables, fruit, grains etc, then you have no obligation or need for animal products. Also, lions exhibit all kinds of behaviour that you would seek to avoid, for instance, violent territorial disputes, and male lions will kill the cubs of a female he wishes to mate with because she won't mate while she has cubs around. Lions are not good ethical role models.
The human brain theory has been debunked over a decade ago. Starch is what made our brains grow to this size. The brain works on glucose after all, not on fat and cholesterol. You say that humans need meat. This is false as proven by all the hundreds of millions of vegans in the world, myself included. It's actually unhealthy for us.