Imagine trying to get back from that.

Hell it could be a good plot.

Dude wants to kill X for something personal, god refuses his plea for the killing is unwarrented, the devil refuses his plea for the motive is honorable, as he laments the situation he is visited by the "god" of chaos who gives him the gun.

Guy goes to the person he hates and shoots him.

Flashes to the middle of a world war, the guy is an enemy soldier, dude freaks out, tries to peace together what he missed when the world changed in that instant.

Flees the battlefields trying to find a way to set right what has happened.

Eventually he fails he cannot ever bring back what was. The moral of the story is that nothing ever stays the same things always change. Chaos occasionally shows up to mock him. (Like lean against a wall eating an apple sort of thing while main char does something. "That won't work you know" kinda thing. The main character kills himself trying anyways... And fails.

Final scene cuts to the chaos god in his etherial lounge, hanging up a picture of that first shot being fired. Talking with War the horseman of the Apocalypse. The picture next to the one of the main charcter is the assassination of arch duke Ferdinand.

Chaos: "See, two out of three world wars each with one shot" War: smoking a cigar "Show off"

Fade to black.