C. Donnie goes back in time and lets himself be crushed by the jet engine, thereby preventing all the deaths he caused.
See? Perfectly simple.
It's just a reversal of It's a Wonderful Life, where instead of the main character deciding to live because the community needs him, the character decides to die because the community is better off without him.
But how did Donnie cause people to die? Well, at one point he gets pissed off at a motivational speaker and burns down his house, accidentally revealing a massive kiddie porn stash there. The guy is arrested, which leaves Donnie's little sister without a chaperone for her dance competition in another city -- because of Donnie, his mother is forced to go along as the chaperone, and on the flight back, the plane is caught in a storm and crashes, presumably killing everyone on board.
Before that, Donnie hooks up with the new girl in school and one night convinces her to go to the house of a crazy old lady -- once there, the girlfriend is knocked out by bullies and run over by a car. She dies, and a distraught Donnie shoots the teenage-driver-in-a-bunny-costume, killing him, too.