There are two reasonable assumptions that imply a surprising conclusion: vegetarianism increases human welfare at the expense of animal welfare.
- Assumption 1: Living things are generally better off existing than not existing. The only exceptions involve constant pain of some kind, and are generally rare.
- Assumption 2: Animals in factory farms may sometimes be worse off than if they were in the wild, but their lives are not so terrible as to be worse than non-existence.
To test assumption #1, I think suicide is the wrong way to think about it, since that has many other cultural associations. Rather, think about reincarnation. Morpheus offers you a red pill or a blue pill. Neither pill will have any effect until you die at the end of your natural life. At that point if you've taken the red pill then you have no afterlife. If you take the blue pill, you get reincarnated as a newborn piglet in an industrial farm somewhere in Utah, where you share a cramped space with 100,000 other pigs. (And after this, no more reincarnation.) Which pill would you take? If you prefer the blue pill, then I would say you accept the above two assumptions. (Apparently, the Talmud does not accept them, claiming that "It would have been better for man not to have been born, but now that he is born, let him look to his deeds." But let's leave this to the side for now.)
In this case, a higher rate of vegetarianism means that fewer animals will be raised and eaten, and as a result the Earth will be able to support more people. (Ok, so assumption #3 is that we reach an equilibrium without blowing ourselves up.) Thus human welfare will be higher, since there are more of us, and animal welfare will be lower since there are fewer animals.
What are my personal views? Well, I agree with the above two assumptions, and I prefer people to animals, so I think that vegetarianism is the more moral choice. However, I'm a hypocrite, so I eat meat.