RE: Why doesn’t Ben kill a defeated Vader?
Jedi Knights are honourable and chivalrous. Killing an opponent in a duel is one thing, but killing a vanquished fallen opponent is something entirely different. That is an execution.
We already see that when Anakin ‘executes’ a defeated Count Dooku. It is a wholly evil thing, and not the Jedi way.
Plus, trauma, shock and disgust. That much was evident from his reaction to Vader’s taunts. He wasn’t ready to see or hear that, even if it brought closure.
With the Jedi being inspired by samurais, I’m also sure he knew it would be more humiliating for Vader to crawl back to his shuttle in that state, rather than being killed in combat by his former master.