Every time the Empire builds a killing machine and places living beings inside it, there's a sacrifice of life that needs to be made one way or the other. If star destroyers (like those in TROS) posed that kind of threat to innocent beings, then blowing those up would still allow a true Jedi to act in the name of defense. There's no real practical way around it.
I think destroying the Death Star is still very much an act of defense to save countless lives, even if Luke *had been* a Jedi when he blew it up. But in ROTJ, he was willing to *be* blown up with DS2, rather than murder bad guys and flee.
Luke can kill living beings, even after his moment of ascendancy in ROTJ. But if he does it, there needs to be a sense that he has no other choice. It shouldn't be just "if you're on the wrong side he'll kill you." Or do you actually believe that Luke post-ROTJ throne room scene would be running around killing everyone on the wrong side?
To me, it would need to be a scenario consistent with Yoda's teaching about "only for defense, never attack," and where he can't use the Force to resolve the threat in a non-lethal way. Otherwise, what was it he learned?
What would be the purpose of tossing his saber and not killing Palpatine when he was convinced the Rebels weren't going to succeed?
His moments of violence on DS2 came when his emotions were stoked and he felt desperate to save people (Palpatine taunting "strike me down" and Vader taunting "perhaps she will"). Those examples are the essence of "fear is the path to the dark side." A Jedi will always be tempted to act that way and give in to fear, but a true Jedi can overcome the fear and not give in. That's why Luke only became a true Jedi when he made the choice he made at the end of ROTJ.
Don't know how GL's intent there can be anything but clear.