Hands down, as far as I'm concerned, The Texas Chainsaw Massacre (1974) is the greatest pro-vegetarian movie of all time. If all that you know about it is that Leatherface kills kids and has a weird family, go back and watch it.
At the time it came out, it was said to have many connections with Nixon, Vietnam, etc, but when the director, Tobe Hooper was asked what it was about, he simply stated, "It's about meat."
To view it just as a "horror" movie is to miss Hooper's entire point. It is a morality play (as most horror movies actually are), with the lesson being that meat is meat, and meat is murder, no matter what the source.
Early on in the movie, the teenagers are seen driving through Texas, where they come across a horrible stench, which could only come from a slaughterhouse. While one character is describing how the cattle are killed, another plays the part of the willfully ignorant public, crying out, "I like meat, please change the subject."
When Leatherface enters the film, we see him treating the kids how we treat slaughterhouse animals. My favorite is when he sticks one of the girls on a meathook:
And thus the meat-eaters are turned in to meat themselves. Also of note are the scenes of torture, when the humans are seen in pain and yet we hear sounds of cows and pigs instead of human screams. Amazing. Go and re watch the film, and make sure your meat eating friends are there to see it.
