A Vegan dilemma - To kill or not to kill

A few weeks back, I was involved in an agitated debate with a friend on whether the consumption of meat by humans is justifiable. He was interested in this question from a philosophical/ethical standpoint, while I was interested in pushing my agenda of vegetarianism. Hence, it was not an impartial debate from my perspective. Over the course of two hours, our discussion arrived at several islands of thought such as the definition of sentience, the ability to subjectively evaluate one's environment, the ethical implications of humane meat farming and so on. For instance, we spent close to an hour arguing whether we can categorize animals on a spectrum/scale of sentience. He argued that cats/dogs are vastly more sentient than ants, while I countered that ants can collaborate and build architecturally complex ant-hills, which cats/dogs cannot.

All said and done, I would like to believe that I held my own during the debate and there was consensus on a few major points:

1. From a health perspective, growing scientific evidence proves that vegetarianism/veganism is an ideal lifestyle.
2. The depredations of the large-scale animal farming industry (especially in the USA) are ethically unjustifiable and ecologically unsustainable.

However, we found the debate repeatedly veering towards two questions, which I could not satisfactorily answer at that time. The questions are:

1. If/when artificial intelligence is finally realized, will the ethical restraints on harming/murdering living things apply to robots/artificially-sentient beings?
2. Is a Vegan/Vegetarian morally obligated to stop 'all' killing of animals - including the killing of animals by other animals? Hypothetically, should an ethical vegan try to 'veganize' all carnivorous animals?

The first question is quite profound, and I don't think anyone has a definitive answer for it yet. Technology, especially machine intelligence, is so new and developing at such a breakneck speed that few have stopped to think through its ethical implications. As movies like Her have shown, humans are susceptible to personifying newer forms of advanced machine intelligence. Heavy users of Siri would agree with me when I say that She (or He) seems remarkably human-like, quirks and sarcasms included. I outright refused to entertain this question on the grounds that I have not sufficiently thought about this problem, and it does not necessarily intersect our primary topic of discussion. It was a weak defense, but the honest one.

The second question rattled me. If true, it threatens to hurtle the ethical argument of Veganism towards an absurd conclusion. Vegans try to live in harmony with nature, and yet the ethical implication of Veganism would suggest that we act to reverse the natural instincts of other animals. I tried to argue against this implication in a few different ways, but I don't think I cogently put forth my views. I hope to do so in this post.

The ethical argument of Veganism is not that all killing is bad, but rather that the killing of animals, by humans, for human consumption is bad. By consumption, I'm referring to all forms of consumption, including food, cosmetics, etc., although I'm primarily concerned with the dietary consumption of meat in this post.

There are a number of reasons for this stance:

1. Physiologically, humans are almost completely herbivorous. We do not have the biological structures or emotional instincts of a carnivore. Our nearest evolutionary relatives are predominantly herbivorous with occasional consumption of insects or even rarer, small mammals. 
2. Human consumption of meat is mediated through slaughter-houses, which are nothing but concentration camps for animals. Anywhere from 50 to 150 billion animals are slaughtered each year through these concentration camps. Considering that a sizable chunk of world population (~7 billion people) are predominantly vegetarian, the numbers are astonishing. The animals are sheltered and slaughtered through methods which would put Jigsaw to shame. Such large-scale murder through inhumane methods is ethically unjustifiable to Vegans.
3. There is no nutritional benefit of eating meat that cannot be obtained from plant-based sources of nutrition. This ties in with the first point about how humans are naturally built to be herbivorous.

There are a number of other non-ethical reasons - such as the environmental impact, the economics of agriculture etc. - that augment this stance against the killing of animals by humans for human consumption.

In my opinion, killing of animals/life-forms is OKAY to a Vegetarian/Vegan under certain circumstances:

1. When there is no food/form-of-sustenance available apart from meat. The desire to survive is a more primal instinct than ethical compulsions, as captured in Maslow's hierarchy of needs. If you're a Vegetarian/Vegan and disagree with this point, I respect your individual stance, but I don't think it applies to all Vegetarians/Vegans.
2. When the animal represents a risk to one's health and safety. This is the justification for a Vegetarian pasteurizing his milk or taking medicines to kill bacteria.

Alas, during the debate, I ventured towards equating Veganism with the belief that killing is always bad. That led me down a slippery slope and it was hard to save face after that. The notion that 'all killing is bad' is a beautiful belief but it is very hard to defend. Jainism has a rich history of investigating this question, but I'm not privy to their deliberations or conclusions. I don't wish to elevate Veganism's ethical restraints to the rigors of Jainist beliefs. That is unnecessary for the more modest goals of a Vegetarian/Vegan.

A few references:

1. Humans are not omnivores
2. A comprehensive argument for Veganism by Gary Yourovsky
3. Earthlings - a documentary on meat industry