Guilt
Guilt: A feeling of responsibility or remorse for some offense, crime, violation, or wrong, whether real or imagined.
Force, he had imagined that the agony would abate when he died, but it only continued. Oh, the physical pain was gone, of course, but the emotional pain endured. In fact, it was more powerful now.
Now, he knew for sure that he had selected the easy path when he joined the Dark Side. It was only the good beings who felt guilt. A Sith could choke anyone who displeased him without a qualm, but a Jedi couldn't. A Sith could blow up a peaceful planet without a thought, but a Jedi couldn't. A Sith could blame somebody else for his problems, but a Jedi wasn't allowed such a privilege.
Still, despite how painful guilt was, it was an honor to feel it. Guilt meant that redemption might be possible for him, because while there was guilt there must still be hope. No, he wasn't naïve enough to believe that killing Palpatine alone would be enough to atone for his crimes, but surely it was a step on the right path. After all, he had brought the Force back into balance again with his actions, and he had ended a reign of terror.
Perhaps that was enough to ensure that Obi-Wan would b e willing to talk to him in the afterlife, because it felt like there definitely was one, and maybe the good deeds he had done as Anakin Skywalker would even out the crimes he had committed as Vader and allow him to be close enough to his real Master to send a message of some sort. If not, he could devise a way to communicate with Obi-Wan somehow, since he was an expert mechanic if he was nothing else.
At the moment, nothing seemed more important than speaking with Obi-Wan. He had an overwhelming compulsion to apologize to his true Master for disappointing him, for being an angry, arrogant, defiant, and disrespectful student, and for never considering Obi-Wan's feelings before. Nothing mattered more than explaining that he finally realized how wrong he had been and how cruel he had been to believe that Obi-Wan hadn't possessed emotions as complex as his own just because he had the sense to control himself as Anakin never had the strength to do.
Informing Obi-Wan of his epiphany wouldn't really make up for all the innocent blood he had shed, but nothing could. Maybe the ultimate redemption would lie in him simply comprehending that nothing could compensate for his actions. Who knew? The Force worked in mysterious ways, after all.
All the Chosen One knew as he finally merged with the Force at last and yet somehow seemed to retain an awareness of himself was that if he found redemption, his son would be the first to hear about it, so that he would not be ashamed that he had followed in his father's shoes and became a brilliant pilot and a practitioner of Form V. Truly, he hadn't been lying when he insisted that there was good in Anakin Skywalker even when it was eclipsed by the blackness of Vader, and his son, who had never lost faith in him even when he had lost faith in himself deserved to know that.
Yes, he was finally ready to accept his guilt and his shadowy emotions at long last instead of fighting them or surrendering them, and he only wished that he had listened to Obi-Wan and learned how to do so sooner…