That was always a hole in the story for me... If HAL was in charge of everything then he could have just opened an airlock and taken care of both of them at one, without the need for an elaborate scheme to get them outside.
It did not seem logical to me that the computer would decide to kill then and then make up an elaborate deception to do it.
The book has it different than in the movie.
Poole dies from a suit breach, and then HAL does open the airlocks to try to kill Bowman.