HAL didn't malfunction. He followed the orders that were given him. Unfortunately for his human crew, his original mission orders were added to at the last minute by some secret and poorly thought-through orders, and HAL resolved the conflict in accordance with orders he was given. As a computer, he performed perfectly.
If HAL knew that ""[human] Performance decrements, memory deficits, and loss of awareness and focus during spaceflight may affect mission-critical activities" , it would only support his decision to remove unreliable factors (humans Poole and Bowman) from the situation.
Quite why HAL killed the sleeping mission specialists, I don't know. They couldn't have interfered with HALs successful completion of the mission if they were hibernating.