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.
So, saddened by all of this horror, Donnie manages to travel back in time to the beginning of the movie and decides to just stay in bed and let the jet engine crush him, saving everyone else. The final scenes indicate that, with Donnie out of the way, everyone will live.
Frank goes on to a successful career as a massage artist.
Wait a Second ...
Or, here's another idea: How about not being crushed by the engine and simply using the knowledge gained in the "future" to prevent those deaths? Because we're not sure that dying fixes everything.
For example, if Donnie never burns down the motivational speaker's house, that means his kiddie porn stash is never discovered and he continues amassing his collection and working with children. If Donnie decides to use his time travel as a do-over instead of a suicide, a quick anonymous call to the cops could have taken care of that.
This might have been avoided completely if he'd taken Donnie skydiving.
And then there's the horrifying fact that, regardless of whatever nonsense the crappy knock-off sequel was about, Donnie's little sister is still going to die. Sure, he saved his mother from chaperoning the dance team, but his sister is still going with the original chaperone (kiddie porn guy's #1 fan) and dying in the plane crash. Let's say Donnie's death changes the plans and the sister doesn't go: There's still a plane full of little girls who'll die. Maybe Donnie could have prevented that or maybe not, but he could have at least tried.
And finally, he could have warned his girlfriend that her mom was going to get kidnapped by her crazy ex-husband, which is never fun, and let's not even go into the Smurfs debate between Donnie's friends that will tragically go unfinished. Basically what we're saying here is that paranoid schizophrenics make really bad decisions.