One intrepretation I saw of this was Vader used Battle Strike, spending a Force Point to get the extra 2d6 damage, then made an unarmed attack on the Emperor, spending a Destiny Point to make it an automatic critical hit. He rolled an obscene amount of damage (maxed out his bonus damage dice, which under SECR apparently get doubled along with the regular damage roll), and the GM ruled was enough to drop the Emperor, then applied the creative description of the old geezer literally getting shafted.
Kinda like how Han's "wild swing" at Boba Fett was, in game terms, enough to drop the bounty hunter, but the GM gave Fett a wild, over-the-top death sequence instead.