can anyone help me with a murderer's name?

He was from america, he abused animals (cows, sheep, cats, dogs), and kept their heads as trophies. He was sentenced to execution, for killing a 13yr old girl and 14 yr old boy (both step brother, and sister). He killed them with a gun. He also cut off the girls hands and head. The bodies were found two days after the murder. The police found the animal heads in the woods, with a book he named something like 'the slayers book of death', in which was used by the prosecution.

Oh and he was caucasian, with an alcoholic dad.

Thanks in advance. :)