Vader considered his words. What he's vowed to his grandson - a vow he intended to keep - and what information he had. "I told Kylo I will not allow Luke to grow into that person. Some of that does mean confronting the misguided teachings of the Jedi regarding fear. But I would not punish him for something he hasn't done, no." He wouldn't allow his child to become a coward to struck out in the dark on a sleeping child.

How that was different than what he had done, well. It simply was.