Killing a student who had turned to the Dark Side is a very Jedi thing to do, from a prequel point of view. Even Obi-wan in ROTJ was saying that Luke needed to kill Vader, and Yoda said that there was no coming back after beginning the dark path. Sure, Luke seemed to grow beyond that in ROTJ, but the concept of killing a relative lost to the Dark Side was the primary conflict of that movie. I think he genuinely wrestled with that question, even at the end. To consider that he could return to that state of mind in a moment of weakness isn’t beyond the pale to me at all.