Killer7 is like the end result of David Lynch and Quentin Tarantino doing acid whilst watching Excel Saga, and deciding to make a game.
You play as hitman Harman Smith, who has seven split personalities, each with their own backstory and gameplay style. Each of these personalities were individual humans in the past prior to their deaths, and explaining the specifics of that would take a thesis - which, helpfully, someone has actually written.
The entire game is stream-of-consciousness, and full of international conspiracies, unreliable narrators, unexplained supernatural phenomena, and a whole plethora of other factors that make it a confusing and disorienting mess, in the best way possible. Figuring out just what the hell is going on is half the appeal.
Its cutscenes are cinematic in tone, and often incredibly disturbing - killer7 really doesn't pull its punches. Considering the content rules on AAO, really can't begin to describe it - but I will link you this scene
, under the disclaimer that it is INCREDIBLY DISTURBING and you should not click it if you are under the age of 18 or weak of stomach.
There's no description that can really do justice to killer7, because there is simply no other game like it. Creator Suda51 is known for creating very bizarre videogames, and killer7 is the most bizarre he's ever been.