From an ethics point of view, could it be possible that an omnivorous diet (which includes grass-fed cattle, chickens etc and no/little grain-based vegetarian foods) results in fewer animal deaths than a full vegetarian diet that includes plenty of grains? Reason being grain fields are not devoid of animals (rabbits, rodents etc) which are killed when grains are harvested.
Vegans/potential vegans should also be cognisant of the increased risk of depression if certain nutrients namely, omega-3 fatty acids, from animal foods are not replaced. Some sources say that even then, plant-based omega-3 is not as easily converted and absorbed by the human body. Not saying that the lack of omega-3 is the sole reason for depression but is a potential factor. This is also from personal experience.