Some strong scientific studies indicating such would be a good place to start :-)

On the other hand:

(1) "meat-based diet is actually the healthiest way to eat"

Position of the American Dietetic Association: vegetarian diets, (2009): "It is the position of the American Dietetic Association that appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases. Well-planned vegetarian diets are appropriate for individuals during all stages of the life cycle, including pregnancy, lactation, infancy, childhood, and adolescence, and for athletes."

Health effects of vegan diets, Craig, W., The American Journal of Clinical Nutrition, (2009) From the summary: “Vegans are thinner, have lower serum cholesterol and blood pressure, and enjoy a lower risk of CVD. BMD and the risk of bone fracture may be a concern when there is an inadequate intake of calcium and vitamin D. Where available, calcium- and vitamin D–fortified foods should be regularly consumed. … Vegans generally have an adequate iron intake and do not experience anemia more frequently than others. Typically, vegans can avoid nutritional problems if appropriate food choices are made. Their health status appears to be at least as good as other vegetarians, such as lactoovovegetarians.”

(2) meat can be raised in humane, environmentally-friendly ways

Reducing food’s environmental impacts through producers and consumers. Poore J. and Nemecek T. Science, (2018). Example conclusion: meat and dairy provide 18% of calories and 37% of protein, but uses 83% of farmland and produces 60% of agriculture’s greenhouse gas emissions.

Analysis and valuation of the health and climate change cobenefits of dietary change. Springmann M. et al. PNAS (2016)
“The food system is responsible for more than a quarter of all greenhouse gas (GHG) emissions, of which up to 80% are associated with livestock production. Reductions in meat consumption and other dietary changes would ease pressure on land use and reduce GHG emissions. Transitioning toward more plant-based diets that are in line with standard dietary guidelines could reduce global mortality by 6–10% and food-related greenhouse gas emissions by 29–70% compared with a reference scenario in 2050.”

And well, I'm not sure if humane slaughter matters to conscious animals. I certainly wouldn't appreciate if someone humanely slaughtered me. And as for the consciousness aspect: The Cambridge Declaration on Consciousness 2012. A group of prominent neuroscientists created The Cambridge Declaration on Consciousness, in which they state their support for the idea that animals are conscious and aware in a similar way as humans. “The absence of a neocortex does not appear to preclude an organism from experiencing affective states. Convergent evidence indicates that non-human animals have the neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors. Consequently, the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness. Non-human animals, including all mammals and birds, and many other creatures, including octopuses, also possess these neurological substrates.”

EDIT: these points are cribbed from a blog post I wrote on this subject: https://medium.com/@colorado.j.reed/veganism-from-an-enginee...