"Good, I can feel your anger," the Emperor chuckled. "I am defenceless. Take your weapon! Strike me down with all your hatred, and your journey to the Dark Side will be complete!"

"That's not actually how that works," Luke replied.

The Emperor scowled.

"It is if I say it is!" he snapped. "Who is the expert on the Dark Side here, you or me?"

Luke shrugged.

"I'm a freedom fighter," he said. "In case you missed it, I blew up an Imperial superweapon four years ago – and I know a lot of the people on board weren't actively fighting back against me. But you're the Emperor, literally the leader of an empire which commits atrocities on a regular basis. You're actively taunting me with how evil you are right now."

"You are a rebel and a terrorist," Vader pointed out, in a way that sounded like he was trying to help.

"I don't remember launching strikes because they'd terrify the general populace," Luke countered. "I remember launching a lot of attacks on legitimate military or government targets, though."

He indicated the Emperor. "He's a legitimate government target – and a legitimate military one. He's the commander-in-chief, a military position, and his military is actively engaged in combat operations."

"He is the civilian head of the government," Vader countered. "A civilian position."

Luke shook his head. "There is no civilian government in the Empire right now, it was dissolved four years ago. That's why Leia's out of a job – and that's why the Emperor is a legitimate target."

"I can feel your anger," the Emperor said, smugly.

"Yeah, because you're deliberately causing it," Luke retaliated. "That's not how this works, I already said. You can't just do evil things to make a Jedi angry and then point out that for the Jedi to kill you for it would be evil because you're not using the Force to actively fight back right this moment."

He shrugged. "It's like how war crimes law considers both the killing of civilians and hiding behind civilians to be war crimes. The second one is to prevent people taking advantage of the first one."

"Are you splitting hairs to convince yourself that what you are doing is moral?" Palpatine asked, tauntingly. "Good… good. That's how it starts."

"No, I'm not splitting hairs," Luke replied. "This is an incredibly open and shut case. My father can choke people with his mind from the other side of a room-"

"The other side of the star system," Vader announced.

"-thank you, Father," Luke replied. "And you're his boss. And a powerful Sith, as well."

He began ticking points off. "You're a legitimate governmental target. You're a legitimate military target. You're trying to hide behind how you're defenceless and taunting me to kill you, and even if we discount your actual significant Force powers my lightsaber is right there next to your hand. And if my being angry about it meant that it would be evil to kill you, then there's no means of recourse if people are just more and more evil."

"I assume you have a point," Palpatine said.

"Yeah, actually," Luke agreed, and kicked Palpatine in the face.

His foot actually missed by almost half a meter, but Luke's pinpoint use of Force Kick meant he hit anyway, and the Emperor crumpled to the floor.

Then exploded.

Once the explosion was over, Vader picked himself up and examined his son.

"You do not appear to have fallen to the Dark Side," he said.

Luke brushed himself off. "It would have been worse to let him live," he said. "For all the reasons I mentioned. And he wasn't unarmed, he still had my lightsaber."

He saw his father's minute examination, and spread his hands.

"I gave him every warning," he pointed out. "Now, are we going to get the Imperial navy to stand down, or are we going to have to wait until my friends down on the forest moon disable the shield generator and launch an attack run on this thing's power core?"

AN:

Luke has put some serious thought into this.

And once he's decided you've had enough warning, well… see the opening of this very same movie.