D*MNIT Donnie Darko, last time.

[James]

I'll take this from Movie Mistakes:

Q. Why does Frank go back in time anyway, if it will result in him dying(saving Donnie) and how did HE time-travel? And why does he urge Donnie to do all those violent things?

A. Frank never goes back in time. In a special feature on the DVD called The Philosophy of Time Travel, there is an extensive discussion of what happens when an object slips out of the proper time continuum through randomly occurring portals. Forces exist to ensure that the object has a human guardian, whose responsibility it is to return the object to a portal in time that will send it back to the proper continuum, often sacrificing the life of the guardian. Frank, in the movie, is both a rather unimportant human figure and the adopted face of the force guiding Donnie to his destiny (returning the airplane engine) that exists outside of either time continuum and can speak to the inhabitants at will. The only thing he directly tells Donnie to do is to burn down Cunningham's house, which results in Cunningham's trial and causes Rose to take Kitty's place escorting the dance team to LA, and Rose chooses to take an earlier flight home. This is the only way the airplane engine would have been in the portal to be returned to the proper continuum. The other violent things Donnie did were merely satisfying his own issues with school, his girlfriend, and her death.