I suppose this is a bit EU, but do any of you know how common it is for species to be immune to jedi mind tricks? We know of the toydarians and the hutts, but are there others? If there are only a handfull of known species with this immunity, I would assume that all jedis would learn about them. If there are hundreds, it would be understandable that one didn't know of them all.
Technically, there is no such thing as a Dark Side 'power', I mean there is just the Force, and how you use it. Choking is just using telekinesis to strangle someone.
But there are just things Light Side Jedi won't DO with the Force, whereas someone as callous as Vader would, focussing his hate in order to use the Force to directly kill someone.
Meanwhile, Luke a. didn't kill the guards with it and b. he was meant to be morally dubious in ROTJ.
Like Ush said, we weren't meant to know for sure if Luke was going to stay on the good side in ROTJ. He's dressed completely in black and uses the choke hold which we saw so much from Vader in ESB.
Everything looked like he was on the edge right up until the last scenes where he threw his sabre away.