"Take your Jedi weapon. I am unarmed," the Emperor crooned. "Strike me down with it!"
Vader looked between the Emperor and Luke.
"I've been wondering about that," he said. "Why exactly do you want my son to kill you?"
"I'll possess the body of my killer," Palpatine replied. "I quite fancy trading in for a younger body, though I'll be satisfied with a young apprentice or a dead Jedi. Keep up, Vader."
"Interesting," Vader said. "So, hypothetically, if I were to kill you, you'd end up in a body that's been crippled to the point that it can only function with cybernetics? And your only means of defending yourself against any attempt to do that would be to electrocute it in a way that would cause those cybernetics to shut down?"
Palpatine frowned.
"That doesn't sound very hypothetical," he said.
"That's okay, it isn't really one," Vader replied. "Luke said some very interesting things to me last night and I've been thinking about why I got into this whole Dark Side thing in the first place. It's been novel having someone think the best of me, and you turned me into a perfect killing machine."
He folded his arms. "Luke. Don't listen to him. I'll kill him before you can reach him. In fact, just say the word and I'll kill him, though it may take a few seconds."
Palpatine and Luke both looked confused, in what was probably the first time ever they'd had the same emotions about something.
"I'll have to wait to actually land the fatal blow until I'm in mid-air having jumped into that pit over there," Vader explained. "Or possibly I'd have to cut through that window to send us both out into space, I'm still workshopping it."
Palpatine scowled.
"I sense from your feelings that you are displeased, Master," Vader said. "Is this because you expected me to not remember that time in your office?"
He glowered. "Congratulations. I have found a thing worth dying for. The only reason you're not dead yet is in case my son wants something else."
AN:
Vader's got a simple morality. Perhaps too simple.