While watching the film for the first time, I always thought that it was the Monolith that accelerated the evolution of AI, just like it accelerated the primates'. One thing that could support this theory that HAL, being a computer, revealed to be able to express emotions at the end. Sure, the 9000s are built quite long before the humans even discovered the Monolith, and are already proven to have a perfect track record. But having a perfect track record doesn't mean he also perfectly mimic the human mind. That deals with logic and analysis, but not feelings such as fear, suspiciousness and regret.