A great question to ask Kenny, nice one! Most people don't really understand or want to understand much of the reasoning behind a vegetarian or vegan diet.. I'm happy to contribute as i have a thing or two to say! I grew up eating meat, and almost no vegetables. Since 25 years im mostly vegetarian after discovering how to cook using spices and visiting India for the first time.

Morality (Is it moral to kill when it's not necessary to survive/thrive?)
This question is well put. There is a HUGE difference between eating ethically sourced meat once in a while or eating 'tortured' meat with every meal.

I eat meat once in a while, or when im ill i often like chicken soup. When i buy meat i always buy the most ethical brand I can find and also buy local meat when possible. I consider that OK morally.

All of life is about birth and death. Most animals kill to survive, and the deeper question looms as to whether it is any more acceptable to kill a plant than an animal?! Life is life, and some people do seriously ask this question. I think to be 100% morally perfect we should only eat the parts of plants that have been designed to be eaten.. like animals do.. without killing the whole plant. Examples are most vegetables such as pumpkin, cucumber, lettuce etc.. and all fruits.

Health (Which diet is better for the human body?)
I have studied this and learned a few intersting things about our digestive system and how it handles meat. Our stomach has different enzymes and chemicals needed to digest meat or vegetables, and our intestines are very long indeed compared to true carnivores like dogs. This is because our intestines are primarily designed to digest vegetables over longer periods of time. Many people have several kilos of undigested rotting meat in their intestines because they eat too much of it. We are actually not designed to eat a mostly meat diet! Our canines are pretty lame compare to a dog, and most of our teeth are dedicated to grinding food as we do when we eat vegetables.

Also, due to bio-accumulation, toxins and hormone mimicking chemicals are at much higher concentrations than found in vegetables.. especially in fish! One animal eats so many vegetables, and the toxins get stored and build up in the animal fats that people eat.

It's well accepted that eating a balanced Vegetarian based diet is much better for your health and vegetarians generally have a lower risk of developing high blood pressure, several forms of cancer, heart disease, diabetes and obesity because these diets are usually lower in fat and higher in fiber.

Environment (Which diet has a lower negative impact on biodiversity, water quality, and air pollution?)
It is fair to say that there is NO comparison when looking at which diet has less impact on the environment. The list is extremely long and concerning when you compare the two! For instance, a single pound of beef takes, on average, 1,800 gallons of water to produce. Compare that to 22 gallons for the same weight of Tomatoes! Water is a HUGE issue globally and meat production is a big part of the problem!

There are many more points to make on this, almost too many to discuss them all! Global Warming is our great threat, and the methane produced by centralised mass scale animal farming is a big part of the global warming problem. Methane is around 30 times more toxic as a global warming gas than CO2! The entire industry is responsible for at least 20% of global warming effects. If you take into account the amount of deforestation taking place to accommodate livestock, this figure is actually much more. We are clearing land and forests and losing our biodiversity at an astonishing rate...

Economic (Where is our money going, who are we supporting?)
I don't know a lot about this, and so to put something more positive into the mix I would like to say that some of the biggest USA meat producers are now putting significant money into developing lab grown meat. This new type of very ethical meat is soon to hit our shelves, and in my opinion might be the way forward for most of the world who refuse to give up meat. This new type of meat deals with most of the issues around environmental and pollution concerns, but not health!