I believe the description of the self-destruct device states that it's computer-controlled, presumably to avoid the problem of a sudden boarding preventing manual activation.
You don't have to be a reader of the RISKS Digest to realize how absolutely terrifying that should be for the crew...
HAL9000: Possible intruder detected. Activating self-destruct sequence. 30... 29...
Dave: TURN IT OFF. It was just me coming here, HAL.
HAL9000: I'm sorry, Dave. I can't do that. My orders were to ensure that this ship does not fall into enemy hands. 25... 24...
Dave: ARRRRRRRGHHHHHH!
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-- The thing that goes bump in the night