What I think really happened was that Luke was filled with blood lust and a surge of satisfaction at his victory. Staring at his fist… marveling at his power. And why wouldn’t he be? He just kicked Vader’s ass
Luke Skywalker, Sith Lord
Rob Conery

Except you forgot the entire reason he found himself on the death star in the first place. He’s explicitly there to turn his father to the good side. How’s he going to do that if he himself becomes sith? He’s quite obviously horrified, as everyone immediately assumes when watching the movie for the first time.