The answer is: Steroids...

Disabled man 'killed for sport'

Brent Martin died in hospital after being found with multiple injuries on Sunderland's Town End Farm estate in August last year.

A youth, aged 17, who cannot be named for legal reasons, denies murdering the 23-year-old, who was found dying next to a parked car. William Hughes, 21, and a 16-year-old have already admitted Mr Martin's murder at Newcastle Crown Court.

On Tuesday, at the start of the 17-year-old's trial, the court heard that the attack began at a bus stop when he bet £5 that the 16-year-old could not knock out Mr Martin with his right hand.

Both youngsters were trained as boxers, and their victim was repeatedly punched, kicked, stamped on and head butted, the jury was told... (source)

...The question is, "What would make adolescent boxers bet each other ten bucks to mortally wound the mentally disabled?"