This is a question that we just don't have a straight answer to. Explanations have varied widely, from being a genuine hunting accident to being a ritual sacrifice by a pagan fertility cult.

Historically, Walter Tirel, lord of Poix, has been put forward as the one who fired the shot that killed William. Most of the conspiracy theories have centered around Tirel's flight to France and his relationship to the Clares and Giffords - both families that are said to have benefited from Henry I's rule - as well as Henry's quick response to the death.

C.W. Hollister, in "The Strange Death of William Rufus" did a good job of dismantling the conspiracy theories. He showed that hunting accidents weren't exactly rare, that the Clares and Giffords didn't have any great benefit from William's death, and that it was simply prudent that Henry acted so fast to secure the throne. More recent works have pretty much stuck by this finding.

To sum up, I think the consensus is that William's death actually was an accident, but it's a question that will likely never have a sure answer and so will always be a great target for conspiracy theories.