When I was 10, I spent a Summer in Hawaii with my uncle. One of his friends was a former Rikishi. This is what he told me.
In olden times, it used to be the Sumotori/Rikishi that would commit Hari-Kiri if they lost. If the wrestler lost because of a judge’s mistake, then the judge was honor bound to join him in death.
I very much doubt it, TERRA, although I can’t find a cite. If that were the case, you’d have very few rikishi left at the end of the basho. Or even if your friend meant they’d commit suicide if they had a losing basho (tourney), you’d lose almost half of your rikishi in one go.
There’s part truth and part legend in what you heard, TERRA Rising.
Here is a picture of a sumo referee. Notice that there is something that looks like a dagger tucked in his belt. Refs have been wearing this dagger for centuries now and it’s generally taken to mean that if he makes a mistake, he’ll commit suicide. Of course nowadays if he does make an obvious ruling mistake, he’ll only get a slap on the wrist from the Sumo Association - he might even get suspended. From what I can gather, though, no one really knows if a referee actually ever was forced to kill himself for making a mistake. It might always have been a symbol that meant “I take my work seriously.”
As to Sumo games being like deathmatches, it highly depends on what era and what context you’re looking at. However, the rules have always been that you win of you get your oponent out of the circle or on the ground. I say “circle” because the dohyo is an Edo-era invention. It used to be that the crowd was the boundary. So in the way, the goal was to throw your oponent into the crowd!