vaderios said:
Vader is like the Thief near Jesus Crucifixion. He made a lot in his past life but always at the end when the forgiveness is coming Mr V is finally redeemed and finally free from Palpatine's leash.

-Angel

There's the problem, he makes a sacrifice of sorts (he knows that Palpatine will not die without a fight so he will almost certainly not survive killing him saving Luke) but he knows also that he won't survive long anyway.

Luke just bested him in a duel and if Leia became a Jedi or a Sith she would either do the same or die trying (any way other than killing the Emperor, Palpatine would win).

So his motives are not clearly altruistic and his timing comes across as more out of desperation than out of a genuine realisation of the wrongs he needs to redeem.

If we saw him plotting against the Emperor earlier he would still have a conflict between his Sith urge to kill his master and his Jedi urge to destroy the Sith but he would be a little more active about it and not such a weed.

Vader is such a strong character in ESB but in ROTJ he comes across as inert.