" He has to be intelligent enough to see the ethical implications of his actions, and to be able to determine the probably outcome of those choices."
That's the Padme Rumination scene and him crying on the balcony on Mustafar....
And the reason he does serve the Emperor after Padme is gone is because he has nothing else, but the Emperor. He's a shell of his former self so he can't beat the Emperor. He doesn't have Padme any longer. What else is he going to do?
He starts as a slave. In a roundabout way, he's a slave as a Jedi. And at the end of it all, he's Palpatine's slave. Only when he has compassion and love for his son does he set himself free and end the horror that he has created.