When Luke strikes Vader down the Emperor says to Luke that if he strikes Vader the final blow he’ll take his father’s place by his side meaning Luke would’ve given in to the dark side.
I never believed that Jedi’s kill, they only harm in self-defense with their lightsaber but if Luke had not thrown his lightsaber away and was able to deflect Palpatines Force lightning with his lightsaber and taken the Emperor down would he have still given in to the dark side?