There is another possibility that some hesitate to make. Mankind is physically and biologically primates. We have the same physical body (arms, legs, hands, ect) Mentally we differ greatly, yet within our brains we still possess primal instincts to survive. One common trait among primates IS face eating. A defensive and aggressive display of power. This case is interesting that the attack continued even after being shot. This proves that the act was deep within the primitive portion of the brain separated from conscious thought.