That is not tension, that's a plothole. Tension would be Han and Leia having that drink with Vader in Cloud city and Lando attempting a joke as an icebreaker.

It might work as tension if we knew that Palpatine was the only one who knew of the rule, but that would have needed to have been mentioned previously, and then in the film itself as a reminder. Also lets remember, Vader gives NO suggestion that he is even contemplating kiling the Emperor in ROTJ. He even explicitly states that "I must obey my Master" (Although all these things might be changed etc etc)

In the days before the PT, Luke is instructed to kill his father because he is old and broken, and because the luke we knew from ESB and ANH would have really wanted to do that (hence how his character has developed).