People keep referring to that, but Vader died. It's not indicative of "paragon" status, only some kind of redemption beyond the rules. He's effectively outside the boundaries of whatever Morality system there is. His score doesn't matter. Or to put it another way:
'E's off the twig! 'E's kicked the bucket, 'e's shuffled off 'is mortal coil, run down the curtain and joined the bleedin' choir invisible!! THIS IS AN EX-SITH!!
I seem to recall reading that, in an earlier version of the script, Vader's decision to kill his boss (who, in that same script, he'd already been plotting to murder anyway) didn't fully redeem him; but did bounce him back enough that Ben was able to intervene at the moment of his death and keep his 'spirit' from being consumed by the dark side (or something like that).