Why was Sidious so easily killed in ROTJ? Well, George Lucas actually directly answered this question once and it's kind of brilliant and touching in a way.

By ROTJ, Palpatine had become an epitome of darkness. He was so consumed by the dark side that all he could feel was anger, hate, fear, etc., and he had mastered these emotions. Lucas said if Vader had attempted to kill Palpatine out of anger, greed or lust for power, Palpatine would have known it instantly. However Vader's actions weren't motivated by any of those; it was motivated by love for his son, and love was an emotion Palpatine had no understanding of. Therefore, because he acted out of love, he was able to take Palpatine by surprise and kill him before he knew what was going on.That's great writing. Return of the Jedi is one of the best SW movies.​ via /r/StarWars https://ift.tt/3rj0yhJ