Well one thing that should be accounted for is that Vader never really had a stable personality. He was constantly torn between his desires, his impulses, and the way of the Jedi. It think for him suddenly to have a one track mind just because he gave in to the impulse to save Palpatine from Mace is a little iffy- because even in that scene he was torn and immediately regretted what he had done.
Perhaps Vader felt a little helpless upon his betrayal of the Jedi, as the RotS book said that he couldn't bring himself to kill Palpatine in the very end because he was the only one left who would accept him. I suppose that could change him, but at the same time, he must have always harbored his hatred for himself and the Emperor. Luke could sense the conflict within in, remember.
"Everything I tell you is a lie. Every question I ask you is a trick. You will find no truth in me."
―Vergere to Jacen Solo