RotJ:
Luke arrives to Vader unarmed, takes Vader’s saber to attack Sheev, Vader defends with Obi-Wan’s lightsaber.
End on Luke burning Vader’s body
Why shouldn’t Luke attack with Obi Wan’s blade?
After all, Obi wan and Yoda ask him to fight Vader and Luke dropping the saber would conclude this conflict well.
On the other hand: what would be the motivation for Vader to fight with the very weapon that made him the machine he is?
Is it him holding on to the though of himself as irredeemable?