I think the idea is that he was a shattered, devastated vessel at this point with nothing left but the Dark Side. His beloved was dead. His order was destroyed, many at his own hands. His old mentor had betrayed him. Ultimately, that void was filled with ambition and the need for power, to rule the galaxy, as was his plan in Empire Strikes Back. There are some hints as to his belief that the strong should rule in his conversations with Padme. This belief is twisted into autocratic visions. "If you only knew the PPPpppppooooowwwwaahhh of the Dark Side."