‘Why did HAL kill the crew?’ is one of the film’s big unanswered questions. Some believe the onboard HAL 9000 computer deliberately plotted to hijack the mission and its date with destiny on Jupiter; others, quoting AC Clarke’s novel, say he was driven crazy by a programming Catch-22.
A simpler explanation: HAL screwed up. After he mistakenly diagnoses an antenna fault, the crew decide to disconnect his higher brain functions. From that point on, it’s a life-or-death battle for survival of the fittest. But that’s just our theory – during production, Kubrick drastically recast the ‘Jupiter Mission’ episode and deleted any clear explanation of HAL’s behavior. It’s better that way.
Did you notice? After HAL’s error is discovered, Frank Poole tells Dave Bowman: ‘I’ve got a bad feeling about him’. It’s the film’s first reference to feelings. Facing a threat they only vaguely understand, the Earthmen regain that old ape intuition – and become human again.