i'm not a vegetarian, but a few of my aunts and uncles are. my mom too, is slowly reducing her meat intake as she intends to become fully vegetarian soon. From what i know, my aunts and uncles feel that killing live beings, whether its chickens, fish, cows, pigs, etc, for food is cruel, wrong, unnecessary and will result in bad karma. I think it's more of a religion/belief thing, because my aunt once told me that if i eat meat, i will be reincarnated into an animal in my next life.
I'm still not convinced though.