Did you guys not watch the fight? At what point did Vader express any desire to torture Luke or inflict pain on him? He started the fight by just standing there and blocking Luke’s attacks. His only desire was to put Luke in the freezing chamber and put him to sleep.

As the fight drags on, Vader flings objects at him and hurts him, but I wouldn’t call it sadistic.

Then he finally starts to get pissed off and wails at the kid. Still, even after all of that, he was just trying to corner Luke and make him an offer he couldn’t refuse.

It was only after Luke hurt him that Vader finally dispensed with all pretense and chopped his hand off in about 2 seconds. That will teach you son! Now listen to your papa and do as I say.