system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars V] Why did Vader cut off Luke's hand?

I thought this was a pretty obvious question, but looking through the search bar, I can't find it!  
  
Having discovered that his son is alive and at his location, Vader goads the boy into a lightsaber duel, and bests him into submission easily.  
  
He then mutilates his only (known) family, and demands that he join him. Finally, he simply watches as his own son attempts suicide.  
  
If you only watch the original trilogy, you might believe that Vader is just an automaton, obsessed with power, and so corrupted by the dark side that he has lost all human connection.  
  
But if you watch the prequel trilogy and read EU, you see that none of that is the case. He is fuelled by anger and guilt (not to mention the fact that his even attempting to contact his son shows that the emotionless theory is bunk)  
  
So why did he act so coldly to his son? If he was such a good swordsman, and wanted to show Luke what power he could obtain by joining him, why didn't he stop short of cutting off his hand, and just disarm him? Why the hell did he think that would actually work?
gpt
Vader doesn't know the best course. He knows Luke blew up the Death Star; he knows the Force is strong in him. But he doesn't know *Luke.*

Maybe the best answer is to kill Luke, go back to the Emperor, truthfully report a mission completed. Maybe Luke is a useless distraction that will only get Vader and himself killed if they try to team up in any way.

Maybe the best answer is to hand Luke over to the Emperor, try to turn him. Maybe installing Luke as a Sith Lord Jr. or rookie Inquisitor will further Vader's ambitions more than Palpatine's.

Or, maybe the best way is to keep Luke as a secret weapon, spring him on the Emperor at a critical moment. But this is only even remotely a good idea if Luke is very strong and can stand up to the heartless reality of Sith power struggles.

Vader doesn't know how he wants this to end, how he can turn it to his advantage. Only way to find out: test Luke to destruction. See what he's made of.