I, personally, like how Richard Kelly showed two ways it could be. In the beginning, Donnie survived, but he was a little mentally sick, his mom and sister died, Frank "the Rabbit" and Donnies girlfriend died too, but that pedofile was put in jail. Then, in the end Kelly goes back to past with this move: Donnie opens a gate into the past and the aircraft engine goes with him and drops on him. But it was like a disguise, so we could see what would happen if Donnie died. His girlfriend (who doesn't now know him) lives, his mother and sister live, Frank lives but the pedofile is free.

...at the end when she said she never knew him, after all they had been through.