good point superfly, i think 'love conquers all' is the moral of the story (rotj that is). before vader is redeemed, he plans on corrupting his son and turning him to the darkside, he plans on killing him if he cant get him to turn, and up till that final moment he was pretty much evil...or was he?
the bottom line is that vader could not kill luke. as much as he talked of "he will join us or die" and "if you will not fight, then you will meet your destiny", vader could not do it. the very fact that he didnt kil luke at bespin shows that vader had good in him, as luke pointed out.
only in the end did he turn, when vader had but 2 choices: 1-let your son be killed 2-be redeemed and save your son. but that was the only moment where he did not have the option of keeping his power, he was not able to lie to himself and say "i'm doing it for the good of the empire"
but was a father's attachment to his son enough to kill the sith? what of a son's attachment to his father?
if luke was raised in typical jedi fashion, and never given a loving family, but instead spent his whole life being trained, would he have reached out to vader? would he feel the good in vader? would he make any connection at all?
luke was given a very similar childhood as anakin. he lived on tatooine, an isolated planet where all he had was his family, his friends, and his thoughts to keep him company. he was raised by loving parents who really didnt want him to go off and be a jedi (shmi allowed anakin to chose, but you can tell she didnt want him to be in a dangerous profession) most importantly, luke learned the pain of losing his family in a tragedy, as anakin did.
so, i think the big question is: why was luke alowed to be raised as a commoner? why was he not taken to dagoba to be trained as a jedi by yoda? why was owen and beru given custody of luke, so he could be raised in the very place his grandmother was buried?
were ben and yoda leaving a certain option for the future open?