system
Given the following post, respond as if you're commenting on Reddit:
human
If the emperor was all powerful then why could vader kill him so easily?
gpt
Several reasons:  First, the Sith seem to generally have less control over their force powers than the Jedi, relying more on raw power than precision.  As a result, when Darth Vader attacks the Emperor, the Emperor sprays wildly and is never able to refocus his lightening attack on Vader the way it was focused on Luke.

More specifically, Vader had Jedi powers at one point before he turned to the Dark Side.  We know from Yoda's example in the duel with Dooko that a sufficiently powerful Jedi can absorb and redirect the lightening attack.  Whether the latent good Luke senses correlates at all with the latent ability to use the light side force powers is unknown.  However, it is known that Anakin was a singularly powerful user of the force and if limited cross-morality use of that power is possible, then he would be the one who could pull it off.

Technologically, Vader was basically a man-shaped tank built by the same civilization who brought you the Death Star.  Their lack of design foresight aside, it is reasonable to assume that they probably put in a surge protector.

Finally, the Emperor himself was never really the most powerful of the Sith, that was always supposed to be Anakin.  Granted, the Sith Lord was an exceptional liar and manipulator, but he lost badly to Windu and barely fought Yoda to a draw, and neither of them were The Child of Prophecy.  We don't know to what extent Anakin's conversion into Vader may have diminished his force powers, but the starting premise that the Emperor is "all powerful" seems dubious at best from what we can gather from the circumstances surrounding his rise to power.