Then, because God had already shown them the «right» way, He pulled Cain up on his «non-blood» vegetable offering as not doing what is «right», as it wasn't shedding the blood of an animal (which was essentially a type and shadow of Christ's sacrifice, as well as being the pattern already set by God in front of Adam and Eve in the Garden). (redeeminggod.com)
«The blood of animals could neither «atone» for sin nor bring God and men together. (faith.org.uk)