I am by no means a Donnie Darko expert but I can't resist taking a stab at this enniway (sorry about the bad pun!) Donnie lashed out at the rabbit that was haunting him as a means of rebellion, and also, I think, to see if he could hurt it, thereby proving it was real and not a figment of his overactive imagination (or a hallucination brought on by all the drugs he was taking). And wasn't that Donnie's mother in the movie theater who turn to him and showed him her bloody eye? Revealing to him that the rabbit was actually his inner parent or superego, and he was torturing himself? Lynne, help!!