They don't know it's chicken. But yes they are cannibalistic by nature. If they see a drop of blood on another chicken, they WILL peck the other chicken until it's dead.
I've been raising chickens for 6 years, 30 to 40 at a time. There were times I had to rescue a chicken that was being chased by the whole flock! I have no friggin idea why they do this but they do.
