Murder Suspect No 1

In Hull, a vulnerable man is violently attacked in his home and left for dead after his assailant sets fire to his flat. The 54-year-old is pulled from the inferno by firefighters but is in a critical condition and shows obvious signs of assault. Detectives soon identify images of a suspect arriving and leaving the crime scene at about the right time - then discover audio of a man, believed to be the attacker, taunting the victim. But who is he and what drove him to commit such a crime?