Jonathan Neill, 6ft 6ins and l6 stone, was ordered by Judge Neil Denison to be detained during Her Majesty's pleasure for the 'terrible killing'.
Neill, now 17, but 16 at the time, boasted he stabbed Donald McKenzie as he lay asleep 'because I felt like it. It's a weird feeling to know you are a murderer'.
On Mr McKenzie's 36th birthday, Neill stabbed him 24 times and left a foot-wide cross carved on his back. Later he told friends he would like to kill someone else on their 36th birthday because the numbers would add up to 666 - the sign of the devil.
Martin Heslop, for the prosecution, said that after an evening's drinking Neill had returned to his home in Long Thornton Road, Mitcham, south London, and decided to murder Mr McKenzie.
Eugene McLean, 23, of Rowan Crescent, Mitcham, denied perverting the course of justice by disposing of the knife and Neill's blood-soaked trousers. He was convicted and jailed for two years.