ShaqLevick said:
Unless you don't believe in evolution I'm pretty sure 12 Million years eating simple and complex grains would likely lower intelligence. But that's only based on the previous 12 Million years of evolution, and the proteins created in flesh not found in accessible plant life. Then again who knows the real science of it all.
My first year anthropology course talked about how one of the larger breaks in human evolution came when we started eating meat, which resulted in a signifigantly increased brain mass. I'm not necessarily sure whether or not that would be reversed by going back to vegetarianism, but it's a possibility. Scientifically speaking though, humans have canine teeth/incisors for a reason, the cutting of flesh. Our teeth shows that we are well adapted omnivors, capable and ready to essentially eat whatever food we can get our hands on. Both meat and plants have different nutrients useful for different things.
On the OP, our society has somewhat strict rules of behavior and what contitutes right and wrong. It has gradually developed that knowing something has to die to sustain your life is no longer polite dinner conversation. At a basic level people don't like knowing some form of life steals the energy from another form of life (even plants steal light and water from other plants) every time it feeds.
By basic instinct of survival an organism must always look out for itself in any way it can, helping others is only done if it helps proliferate your species. Since "ugly" animals and plants are so very different in form and nature from humans, we develop very little emotional response to changes in there welfare. "so what if a cockroach dies, they're bug ugly and COULD infest somebodies house".
However, "cute" animals remind often on some basic level remind us (or resemble) ourselves when we were babies. Thus creating strong emotional ties to such an animal even though they are not related to our own survival. Thus is is often thought of as wrong to kill such an animal, simply because we have developed an emotional response to protect such animals like we would our own human young.
While there really is no rational difference between killing a baby seal eating all the fish, and a cockroach eating all your chips, often irrational emotions get in the way, and cause these outcries.