It is wide open to interpretation. I've written extensively elsewhere about my take on this, but the ambiguity is part of the magnificence. The film loses grandeur with attempts at neat answers. This was attempted in the deeply ordinary 1984 sequel 2010, when the explanation for HAL going kill crazy simply turned out to be something as banal as "contradictory programming". My interpretation is far more interesting: HAL coming into contact with the large monolith around Jupiter catapults him up the evolutionary chain before the humans, creating the first true machine intelligence (who is then dispatched).