A reflection
Blood is prevalent in so many types of films that it would take years to fully digest how it is used as an effect and why. Even the Meyer's "Thicker than Water" could only discuss blood symbols in the ancient world with a limited amount of success.
Human blood on the screen is one thing that can make us as viewers immediately relate to a character and have a rush of emotion invested in the movie. Even when we see teenagers in slasher movies make mistakes time and time again and think to ourselves how stupid they must be, we still jolt when their blood splatters. The grotesquery of seeing a cannibal tear apart a man's body affects us, even if the man being torn apart is a despicable figure. Gore is a seminal way that can cause a really visceral connection between an audience and an actor -- no dialog or emotional storytelling needed.
Why do we enjoy these feelings caused by bloody scenes then? Is it our own pent up violence waiting to be released? Is it a simple adrenaline rush? Or is it some deep-down hatred of our fellow man? Is it some psychosexual thing that we get off on seeing the objects of our sexual affection destroyed? Or is it a cultural phenomenon, that we are indulging in something uniquely taboo with other people?