Palpatine Was Ready To Murder Vader With Order 151

Why did Emperor Palpatine create a secret order that would have seen his army of stormtroopers murder Darth Vader at a moment's notice? Was it because he was always on the lookout for a new apprentice after Obi-Wan defeated Darth Vader on Mustafar? Or, was Palpatine paranoid that one day Darth Vader would find out how he manipulated him, and caused all of the pain and suffering in his life in order to control him? We'll find out in this episode of Star Wars Theory.