My answer is more from a personal understanding of <PERSON> than actual canon or facts. But I believe that it was more from a lack of will to live that he became less "smart" as the question is asking. <PERSON> had convince <PERSON> that he had indeed killed <PERSON> in an act of aggression. After his mother's death and the betrayals of the council (In his perspective a betrayal) all he had left was padme', and even before that she was his first and top priority, she was what meant most to him in life. Losing that, what point was there to go on?
However, as I have read in the book Star Wars III: Revenge of the Sith, it describes how <PERSON> lungs breathed on their own because of the technology that was permanently attached to him. Therefor he could not stop living even if he wanted to. (I'm not stating that he was or wasn't suicidal.) I know when I thought that I had lost my wife, even standing up felt like I had a giant weight pressed down on me, preventing me from continuing life. He had lost his reason for living, so why try as hard as he did. 865
What was the point?
His children were also thought, to him, to have died with <PERSON>'. So <PERSON>'s death, mixed with the fact that he thought he killed her, not to mention his own child (as he didn't know that they were twins until facing <PERSON> at the end of Star Wars VI: Return of the Jedi) and then put on top that his mother was dead, his best friend was evil and manipulated him (In the same book it stated that he realized that he had been deceived by <PERSON> and he tried to kill him with the force, but couldn't.) as well as his father figure, <PERSON>, betrayed him (Again, from his perspective) and the jedi (his last family after the death of his mother) had betrayed him... I'm surprised that his lightsaber didn't end up in his gut....
(Edited) I also, recently had a conversation with my wife about this. She said that he also grew up. <PERSON> was, for lack of better term, a young buck and was more rash and impulsive than he was as <PERSON>. As we grow up we think about what we do before we do it more and more. Maturity, more or less. If you even read/watch/play, <PERSON>'s decisions weren't always the right one, indicating that he was making a decision more quickly. <PERSON> was in all technicality the more mature and adult <PERSON>.
I understand that this is a completely different type of answer than you might have been looking for, and of course we cannot ignore the physical agony that he was in constantly, but hopefully it at least helps you. 865