And not to sound like I disagree, but I will reiterate a point I've made before. Palpatine having Force lightning abilities completely blindsided Luke. He did not understand just how out of his depth he was until the Emperor struck. And this was a complete surprise to the audience. A lot of surprise twists are missed when audiences watch the films in numeric/chronological order instead of production order: the surprise twist that Vader is Luke's father is foremost, but the surprise twist that the Emperor could fry a person with Force lightning coming from his fingertips is another.
The twist that the Sith accumulate their masters' abilities by bodily transfer when apprentice becomes master is another similar surprise twist, a danger blindsiding the hero and us. Not realizing the nature of a danger until it's too late to avoid is perhaps one of the most frightening things imaginable.