Either that or Palpy is just a consumate opportunist. He loses his apprentice, sees Skywalker, senses the boy's potential, and grooms him as best he can until such time as Palpy can use him.
Possibly. I'm not entirely close-minded when it comes to that avenue of thought. But over the last decade I've heard a lot of what Lucas has written, said and implied about Anakin Skywalker's rise, fall and redemption and I think his intent was for Anakin to be a tragic character who was destined to fall in order that his son could eventually bring out the remaining good inside him and the prophecy of the Chosen One would be fulfilled. That's why Palpatine made the cryptic comments about the Sith creating life using the midichlorians in Episode III... two movies after we learn that Anakin had no father and was so immensely strong with the Force. Lucas obviously wants us to believe that the Sith created Anakin Skywalker for their own long-term ends and it was always part of his destiny to betray the Jedi and the Republic, convert to evil and assist the rise of tyranny.
It's just that Palpatine was so cocksure of his grip on ultimate power that he failed to foresee Darth Vader's last-minute redemption. The Emperor was overly confident in himself, which blinded him to Anakin's change of heart and spirit on the final day of his life. And that blindness resulted in the destruction of the Sith Order and the bringing of balance back to the Force.
The way I interpreted it is that the Sith is like a cancer. As long as they are just in the background, just plotting, then it's okay. But once they start moving forward and putting stuff in motion, the Force becomes unbalanced. But the Force can take care of itself and does by creating Anakin. Anakin is destined to bring back the balance by cutting out the cancer, which he ultimately does. It's Anakin's entire reason for being.
Now Anakin had the opportunity to set things right in Palpatine's office in ROTS, but he didn't. He chose the selfish path to seek power, ostensibly to save Padme, but ultimately for power's sake. But the Force had a back up plan: Luke. So when Anakin didn't kill Palpy, he had to suffer for a while before Luke showed up. And it is only then that Anakin stops suffering.
Remember the movie Election with Reese Witherspoon? She has a great line in there that sums up the Darth Vader experience nicely. "None of this would have happened if Mr. McAllister hadn't meddled the way he did. He should have just accepted things as they are instead of trying to interfere with destiny. You see, you can't interfere with destiny. That's why it's destiny. And if you try to interfere, the same thing's going to happen anyway, and you'll just suffer."