If Hal lied about the AE-35 unit just to get one of them out there so he could trap them out there, why did he wait till the second time they (Poole) went out to replace it.
Why didn't Hal just trap Dave out there the first time they went out there?
After the first time Poole went out and retrieved the AE-35 they they tested it and found out it wasn't faulty. HAL had mentioned before that he was "incapable of error" so they were very worried..worried enough that they decided to get into the pod and turn off all communications to keep HAL from eavesdropping. However, HAL was able to read their lips and they said they had no choice but to deactivate him. This caused an already paranoid HAL to do anything to keep from being deactivated, which was to kill Poole(2nd trip) and keep Dr. Bowman from re-entering Discovery One.
Ah yes that's right, HAL only got super paranoid after the lip-reading conversation, which was after the first change-out of the AE-35.
So if HAL wasn't paranoid before he had seen the lip-reading conversation, why did he report the AE-35 unit as dysfunctional at all? It clearly wasn't his plan at this stage to kill Dave, because he didn't.
The movie doesn't explain it but the book says HAL was experiencing a programming error(even before the lip reading) becoming increasingly paranoid, so really it was human error. His first act was to falsely proclaim the AE-35 Unit was going to fail.
That's what makes 2001 such a great and interesting movie. It's told in mostly a visual way so it's up to the viewer to ask questions and try to answer them. You can't go by the book...it's a different beast.