how did sheev not realize his own fucking apprentice, who was standing right next to him and who he had repeatedly cucked over the years, was going to revenge-cuck him as he shocked the shit out of luke?
Ultimately it was Vader's love for his son that enabled him to overthrow the emperor, Sheev couldn't see or imagine this power. He believed that the dark side was supreme and couldn't be overcome by "good".
Vader wiped out the entire Jedi order and Padme to turn to the dark side. EU stuff had Palpatine constantly playing mind games with Vader to make sure he was an apprentice bitch. He thought he was far gone already.