What happens if a boxer accidentally kills his opponent in a match?

Demand a re-match.

Nothing, it's mutually consented combat. Ray Mancini killed his opponent by accident, he lost his edge but never faced a penalty.

When you are punching another human being in the face as hard as you can, killing them is not an "accident."  Understand, imbecile?

Usually he wins by knockout. Google boxing deaths. Emile Griffith [or Griffin] actually beat a man to death after he hinted Emile may like men. 

Usually he wins by knockout. Google boxing deaths. Emile Griffith [or Griffin] actually beat a man to death after he hinted Emile may like men. 

Nothing, unless one has put concrete or any item in his glove(s) that is illegal.  It would b just an accident if the fighter comply with the rules.

Legally Nothing. Contract signed

Boxers sign a waiver and it's filed legally before a fight.

Unless the boxer does something during the fight that shows he's trying to kill you like pulling out a weapon and beating you with it the other boxer doesn't go to jail.

nothing , he didnt intentsionary kill and the law has no power in a private match anyway . how many boxer have died in the ring and sent the boxer to prison !!!

The Loser never boxes again - the winner does.