Donnie's aggressive behavior, his increased detachment from reality, seem to stem from his inability to cope with the forces in the world he perceives to be threatening.
18 seconds sound clip from the Donnie Darko movie soundboard.
You can hear this line at 01:02:58 in the Blu-ray version of the movie.
Quote context
[...]
- Thank you for seeing us at such late notice.
- We both felt that it was time for us to come in and discuss...
- What I think is going on with your son?
- Yes.
- I'm not really sure that's a good example. I think he had just cause to insult her.
- Let me just lay out what I believe is happening here.
- Has he ever told you about his friend Frank?
- Frank?
- Yes, the giant bunny rabbit.
- I don't recall him ever having mentioned a rabbit.
- Donnie is experiencing what is commonly called a daylight hallucination.
- This is a common occurrence among paranoid schizophrenics.
[...]