For the most part human against human bloodsport lived and died with the Romans, but a common sight in fantasy or science-fantasy is the existence of gladiator arenas in some form (Hunger Games to name a current popular one). Which makes me wonder, what historical events would best support (in a realistic way) modern day man vs man combat?
People who want to watch and pay
That could be directly (tickets, subscription TV) or indirectly (advertising revenue). I guess there would be enough people who are sick enough to enjoy real slaughter.
Make it so that death is not common. Injuries are expected, crippling injuries do happen, but no duel to the death. There are many real-world sports where pros will ruin their health.
Freedom to take risks
One could argue that an informed, consenting adult has the right to take the risk of arena combat to earn lots and lots of money. Working swing shifts for a wage is bad for the health, too. Others argue that no sane, informed person would make that choice freely -- when anybody tries to become a gladiator, he has to be restrained until he comes to his senses.
Protection from liability
Organizers and contestants would have to be shielded from prosecution for murder or assault. Even if death is not the only outcome, they would deliberately attack another human being with a lethal weapon and the intent to inflict wounds. If that happened in a dark alley, it would be a crime.
For a dystopian, bleak future, I could think of a few ways.
Nazi social and genetic engineering taking hold: Seeing other races as inferior would lessen the ethical dilemmas involved in combating to the death with beasts and each other. Prerequisites for this would include things like Christianity not necessarily having such a huge influence on the culturally accepted views on humanity. Or maybe just Hitler actually succeeding in turning the German industrialization around without going to war with everyone, but only by blaming the Jewish people. Seeing their crawl from a dire economic situation back to their feet by way of genocide could leave the world thinking there's something to their view.
People buying into the scare of overpopulation: Global warming (yes, I choose to use the misinformed, antiquated term deliberately) and overpopulation mania going overboard and instead of simply switching to the most yielding foods(away from meat production, basically) and dialing down the independence on oil, the solution worlds turned to was a culling of the population, either by force or by not distributing the wealth. Downside would be that I'm uncertain if this could've happened in a historical context, but maybe in the past few decades. This could also create a tiered system, where in order for you to elevate your living standards you'd either buy your way up, or fight in the highly lucrative arenas(close to how the Hunger Games and Divergent series do their thing). Rich nations just wall themselves in and let people join with a highly controlled system.
Radical religion rising to a world power: Be it Christianity or Islam, at their most radical, both are capable of terrible things. The opposing side, plus anyone falling out of favor with the pious elite, wouldn't feel safe for long. Perhaps the crusades would have crushed the Islam opposition completely and gained the excess wealth and influence, or perhaps the Ottoman Empire turned their conquests to Europe and triumphed over local rulers. This religious gladiator arena fighting could take either a public center stage as an atonement of sins, or underground purging of the unclean, a seedy underbelly or even government controlled gambling house kind of system. Upside of such an alternative history would also be that there wouldn't need to be any moral shifts: The xenophobia of the era and cultures during those times would have just been cemented further into the societies.
Concept of human rights not evolving: Modern understanding of slavery is quite a different one from a few years back. As is of people that have different colored skin. Maybe the emancipation proclamation never happened and the slave trade continued in the States, but the industrial revolution results in manpower being obsolete, so ownership of slaves simply continues as a brutal bloodsport.
If your looking for historical context just look at video games or movies.
First movies and video games were mostly PG, but as time went on they got more and more violent.
your gladiator combat could be the same way. At first the fight would only be to fist blood, this would make death and injury very uncommon. After the population gets used to the idea of gladiator fights, critics began to complain that the fights are to short and boring.
The gladiatorial industry begins to produce a few fights that instead of to first blood are to yielding. These fight are more violet and have a great chance of injury or death. But still they are to the death yet.
Religious and civil right leaders speak out against this new form of gladiatorial combat. They called excessively violate and immoral. However this only increase the about of people interested in the fight and many go there just to see these thing for themselves. The large about of money that the new fights bring in Leads the gladiatorial industry holding more and more of them. Soon the about of new fights outnumber the old ones.
After a while the someone has the idea to have a fight to the death between the to rival champions. After have both sides sign a contract that saying that they chose to willing participate in a battle to the death, the industry begins to advertise for the battle. By this time there audience is familiar with see death on screen, however all those death have been by accident. The idea of a fight purposely to the death sounds to both intriguing and dangerous, many make bets on wither the industry and the gladiators will actual go through with it. when the date of the battle comes most of the population is watching the fight. It ends up making more money then any other combated before it. After make so much money of course the industry has to make another one.