Because he knows well that those darkside tendencies lead to genocide. Also, Luke didn't go to kill Kylo in his sleep, he looked into him and for a split second considered the idea when he noticed the darkness.
Last Jedi complaints seem to be more stupid than the movie's biggest plotholes.
@Liquid Stuff, yea.
It's like the question "would you kill baby hitler"
And even if you decide not to, everyone would at least consider it, like luke did in this movie.
Its a realistic and human response.
@Liquid Stuff, but why would he even have to think about it? How would he even explain killing his nephew to his sitter, best friend, and the academy the next? The Jedi academy would have turned on him anyway. He should have known Snoke was planting those thoughts. Luke had dark thoughts in the past too, so maybe he should just kill himself.
@Liquid Stuff, thank you! I saw someone say one of the plot holes was that the projection of Luke was using the blue saber that was destroyed earlier... Yah... that's the point.
@ReverseDatGif, well yeah, that moment of weakness was his biggest regret. He exiled himself because of it, and had no intention of doing anything other than waiting to die.
I really didn't like this movie, but I have to admit, if I had what I thought was a reliable force vision of my nephew becoming space Hitler, I'd likely have a similar reaction. Vader had already done a lot of bad shjt and the struggle was to pull him back to the light. With Kylo, Luke was contemplating stopping the bad shjt before it could happen in the only surefire way. Unfortunately, as Star Wars has always shown us, prophecies and force visions always come true one way or another.
@Yourself, also he said it was a brief instinct, he wasn't going to do it, he just ignited his saber because he felt vunerable and he wanted to protect everyone. He was also "going" to kill Vader but took a moment to contemplate his actions and didn't. Seems pretty true to the character.
Proof that old people are out of touch with the younger generation
For some reason I read darkside testicles
Self fulfilling prophecy