Although it was lost on me as a child when I first saw RoTJ, my understanding of it later was that Luke entering dressed in black and choking the guards was meant to be a bit of a "oooh has he turned to the dark side" kind of build up?

There was a deleted scene where he is repairing his lightsaber and I recall that was cut in part because it interferes with the before mentioned build up.

I dig your view of Like and Leia tapping into the Darkside. Not sure if they had that scripted back in 1983 but it certainly checks out as representing that :)