Arrhichion of Phigalia
Killed by winning a wresting match in 564BCE
Arrichion was facing off against his opponent in a Pankration (ancient boxing/wresting), and found himself in a choke hold. He pretended to be weakened, then suddenly threw his opponent to the ground, injuring him severely.
The opponent submitted, and it took the officials a second to realize that Arrhichion was dead too. He broke his own neck, while throwing his opponent.