Well yes, soylent green _is_ people, but you can't just apply existing homicide laws to a business like ours. The entire humans-as-nutritional-sludge industry would be out of business.
@plexus The movie was based on a book by Harry Harrison called "Make Room!, Make Room!" and was a cautionary tale about overpopulation. In the book, he talks about eating soylent steaks, which are unsurprisingly made from soy beans and lentil beans. Soy + lentil == soylent. Humans have also had to start farming the sea, so they would eat "weed crackers", which were seaweed.
I'm not sure they used much else from the book.