Over a billion people could be fed with just the grain that is fed to livestock in United States alone. (cattle, pigs, chickens, etc)
About 8 million people starve to death each year, so that's somewhere around 4 million females (women, children, babies) getting killed off each year due to the inefficiency of 'most' non-vegan diets.
The only exceptions I can think of for non-vegan diets that aren't as harmful, would be people who randomly fish (non-commercially) & people who hunt for their own food. Not that I'd want to support killing animals, but non-commercial fishing/hunting is less damaging to the human population and the environment in general.
Vegan Diet
It's pretty simple. High water content fruits/vegetables/root vegetables/nuts & seeds (hemp hearts/avocados/olives/etc for those who have nut allergies). Cinnamon helps regulate blood sugar levels & citrus (like hot lemon water) aids with the digestion of lipids.
Making sure to get in plenty of clean water & making sure to get in the recommended amount of salt per day (preferably sea salt), as salt is extremely important for digestion (especially for digesting fats). People who exercise need more salt & water on a daily basis.
www.saltinstitute.org/wp-content/uploads/2013/09/Second-QTR-copy-of-digest-absorption.pdf