Luke is either a replacement for Darth or a replacement for Palpatine which Palpatine has to remove with Darth.
Having Luke watch while his friends are killed is meant to goad Luke into action so what imperialscum says is true ... from a certain point of view.
After the Rebels are gone and the Death Star is completed other things will rise against the regime (probably from within it).
Palpatine hopes Luke will be able keep hammering down those nails when they pop up.
Vader hopes Luke will be take his place as second in command with himself as the new Emperor.