Its very unfortunate but it has happened a number of times. I can recall a few.
Perhaps the most well known death in the ring was that of Duk Koo Kim. He was a Korean lightweight contender who fought Ray Boom Boom Mancini in 1982. Both fighters were pure offensive style fighters. There was little defense in the bout. Mancini knocked Kim out in the 14th and Kim never regained consciousness.
It was a sad loss for boxing and a sad loss for Mancini as well because he was always a well spoken gentleman and a credit to the sport. The loss effected him deeply as he never felt any ill feeling towards an opponent.
Also in the early 80s, Middleweight contender Wilfred Sypion fought Willie Classen. Classen died due to injuries during the bout.
Sugar Ray Robinson knocked out Jimmy Doyle in (1947 I think). Robinson didnt want to fight Doyle because he dreampt, the night before, that something bad had happened to Doyle. The commissioner convinced Ray to go ahead with the fight and Doyle died after being knocked out by Robinson in the 8th.
Ezzard Charles fought Sam Bouardi who died after injuries suffered during the bout. Charles was so deeply effected by the death that he was never the same as a fighter.
Jimmy Garcia died during a lightweight bout in 1997.
Max Baer also fought an opponent who died shortly after the bout.
Emile Griffith fought Benny Peret, a fighter he clearly didnt like. Before the bout, Griffith threatened to kill Peret for calling him 'gay'. Peret died after being stopped by Griffith. Clearly Griffith's pre-fight threat was more from a competitive stand point and not really meant to be carried out.
There have been others but these happen to stand out in my mind at the time. Its something I hope never to see again.
*Notice I dont say a fighter ''killed'' another in the ring. I dont believe any boxer intends to actually Kill another. *