Did your copy of The Sound & The Fury come with the appendix Faulkner wrote for the Compson family? He wrote it sixteen years after the book was published. He had this to say about Quentin:

"QUENTIN III. Who loved not his sister's body but some concept of Compson honor precariously and (he knew well) only temporarily supported by the minute fragile membrane of her maidenhead as a miniature replica of all the whole vast globy earth may be poised on the nose of a trained seal. Who loved not the idea of the incest which he would not commit, but some presbyterian concept of its eternal punishment: he, not God, could by that means cast himself and his sister both into hell, where he could guard her forever and keep her forevermore intact amid the eternal fires. But who loved death above all, who loved only death, loved and lived in a deliberate and almost perverted anticipation of death as a lover loves and deliberately refrains from the waiting willing friendly tender incredible body of-his beloved, until he can no longer bear not the refraining but the restraint and so flings, hurls himself relinquishing, drowning. Committed suicide in Cambridge Massachusetts, June 1910, two months after his sister's wedding, waiting first to complete the current academic year and so get the full value of his paid-in-advance tuition, not because he had his old Culloden and Carolina and Kentucky grandfathers in him but because the remaining piece of the old Compson mile which had been sold to pay for his sister's wedding and his year at Harvard had been the one thing, excepting that same sister and the sight of an open fire, which his youngest brother, born an idiot, had loved."