I play computer games with friends. We tend to go for co-operative experiences, and they're usually shooters. Currently, my friends' main game is Payday 2, which is morally reprehensible in that the players are acting as criminals, cavalierly murdering armies of police, security guards, and occasionally people who just happened to get in the way. However, we have a hypothesis, thanks to my friends Chad and Chuck, as to why it's all OK: clones. Cloning explains everything.
Well, almost everything. We do have to throw in ubiquitous nanotechnological fabrication as well.
Cloning first, though. There is only one security company in this game; GenSec covers security from armed guards, through armoured trucks, locks, alarm systems... anything to do with security, GenSec are the only game in town. All their guards look the same, and they're all very stupid. This is because they're not really people; they're vat-grown drones, given a standard mind image, like a corporate computer. All the security guards are clones.
So are all the cops. There are far more of them than is reasonable (the game is supposedly taking place in Washington, DC, and there have been games where we have chewed through well over twenty percent of Washington DC's entire complement of police on day 1 of a three-day job), they have no regard for personal safety (the criminals have just handily defeated a guy wearing bomb-disposal armour, who was armed with a full-on light machinegun; let's send in a wave of beat cops with 9mm handguns), and they have only three faces. The cops are clones too.
All the bystanders are clones. Somewhat more variation in phenotype, but still very limited, and all the self-preservation instincts of a marshmallow.
The inescapable conclusion is that the players, and some of their criminal associates, are the last remaining natural-born humans, and that they're robbing places to buy food on the black market.
How does that explain the access to ridiculous arsenals of weaponry, though? That's where the nanotechnology comes in. Weapons are expensive, but not because they're intrinsically rare; you can get whatever you want, provided you have the cash. That money is going to the guy who breaks the DRM on your nano-foundry to let you print weapons. The same nano-foundry is responsible for your self-repairing armour, and for your ability to use any ammunition you run past. See, when enemies with guns die, they drop small amounts of ammo. This can be completely incompatible with your weapons, though. I could wipe out a wave armed with nothing but 12-gauge shotguns, 9mm handguns, and 5.56mm assault rifles, while carrying a .410 shotgun and a machinegun chambered for 8mm Mauser, but I still get ammunition I can use from the dropped boxes. The only explanation is nanotechnology. Heck, dropped 9mm bullets can replenish 40mm grenades. It's also possible to be carrying a bag of ammunition, which magically can replenish anyone's ammo pool, despite it being perfectly possible to have eight different ammunition types between the four players.
Clone logic and nanotechnology explain all the gameplay contrivances.