As a vegetarian, I can tell you that a lot of people who believe eating meat is OK do so because they are mistaken about the environment; remove the mistake (by showing them how horrible conditions are in factory farms, for example) and they will see that eating meat is wrong (or at least that factory farming is wrong.) If they genuinely didn't value the pain of animals, say, this would fail.

You wouldn't happen to have non-anecdotal evidence that this is actually the case, would you?