Dubious claim. There is no single argument against eating meat. I know vegetarians who abstain from eating meat on grounds ranging from waste of biomass resources to "I don't like the taste." It's a multifaceted belief system.

Within the group, you'll likely find a small subset that actually thinks meat is akin to human murder, but that takes a leap of logic that many vegetarians aren't willing to take when there are so many other perfectly reasonable reasons to choose the vegetarian lifestyle.