Palpatine's considered a master of psychology--able to predict how his tools will think and plan accordingly.
Vader was tortured by his killing his wife and getting roasted alive on Mustafar. At the end of Revenge of the Sith, the book Matthew Stover wrote, he says that Vader attempted to use the Force to kll Palpatine, but stopped because...
Because he realized that now the Emperor was all he had left of his former Jedi life. Someone who trusted him. I think revenge was heavy on Vader's mind up until he fought Obi-Wan on the Death Star.
My guess is Vader saw SK (then a boy), realized here was his chance to get even with Palpatine for using him all along, and tried to make SK in that vision. Vader tried to make it look like he was doing "only what was necessary" to get SK in a position to help defeat the Emperor and take his place.
Why else would he kill SK and have him revived in secret? Vader tried a simple deception, but didn't expect SK would question his motives and all.
"I lied, as I have from the beginning."
"Not with you, no."
Vader recognized SK was changing and attempted a complex tactic to anger SK into attacking the Emperor, reasoning that if his Secret Apprentice understood Palpatine was pulling the strings all alone, SK would want to destroy Vader's Master first out of revenge.
Figuring this out? Think like Vader and SK. It makes sense to me. Hope this helps.