Imagine a game of Overwatch with no duplicate characters—once a character is chosen no one else can play as them. In this scenario the characters of Overwatch are distinct persons.
In this world the characters of Overwatch are heroes cum mercenaries. Their watch ended, their struggle won or lost, they are reduced to venal pursuits to make ends meet or simply to stave off boredom. Puissant and bereft of a unifying cause, they often find themselves on opposite sides of a conflict. The battles revolve around mundane objectives—moving a payload from one location to another, capturing strategic territory.
In El Dorado and Junkertown a group of Overwatch mercenaries performs armored car duty. Perhaps they are transferring the weekly payroll for Union Pacific, or the ill-gotten gains of a cartel. In Hollywood the Overwatch mercenaries serve as bodyguards for Harvey Weinstein as he travels to the premier of a new Polanksi-Allen collaboration. Another group of Overwatch thugs attempts an assassination; the client remains unnamed.
In this world of violence, good and evil, right and wrong, are alien concepts. All that matters is completing the objective, and the next one, and the money and distraction it provides. Alliances shift with the wind. Your brothers (and sisters) in arms of yesterday find themselves in your sights today and you don't hesitate to pull the trigger.
Of course, in Overwatch there are duplicate characters, which leads me to believe something more sinister is afoot. The objectives are still basic and bloody, transportation and seizing of territory. Yet nothing is ever accomplished. The territory changes hands, the payload reaches its destination or it doesn't, and the same battles are fought over and over again with no conclusion. Death holds no sway; it is an annoyance that delays you a few seconds, just long enough for your ever-shifting team to be defeated.
More disturbing still is the fact that you can face yourself in battle, see an exact clone of your being and be forced to slay it before it slays you and your compatriots. How many times has McCree placed the killing bullet in his own skull; how many misses were due to a flicker of hesitation, a sense that this is all horribly, horribly wrong? How many times has Tracer unloaded the clip of an automatic weapon into her own spine, just to zip away with an insouciant "Wheee!" Surely, Mei, who seems of all the characters to wrestle most with a conscience, must feel a pang of remorse at slicing an icicle through her own heart.
There is an explanation for this unending cycle of violence—the war was lost. Maybe it was the war against the Omnics that the Overwatch characters reference obliquely from time to time. Or maybe it was a latter war against a stronger foe. A foe that conquered the Earth and subjugated or annihilated it denizens, and trapped their strongest exemplars in a simulated environment. Perhaps the heroes of Overwatch are plugged into a Matrix-style supercomputer, forced to battle in a world composed of zeroes and ones. Perhaps they are all Omnics, produced in the foundries of Volskaya, with a patina of skin and implanted memories to trick the consciousness. Or perhaps the conquering race possesses the power to command life and death, and it is full-fleshed organic clones that mash and blast and slaughter one another.
The purpose of this experiment is unknowable. It could be a training simulator, where the strategists of the conquerors incorporate the battle experience of the Earthlings into their own. It could be a punishment for resistance. It could be an experiment in human psychology, discovering how easy it is to draw violence from these creatures, a violence wreaked freely upon their erstwhile compatriots and even their very selves. Most likely the reason is one the hominids comprehend and prize above all others—it's entertaining.