I have to wonder what those saying they want to see Luke fall to the Dark Side, are actually talking about or imagining?
If it's Luke brooding, coldly shoving R2 aside and a couple of Porgs, killing storm troopers (which he's done more than his share), force choking, mind controlling, to get his way, killing bounty hunters, ship full of aliens and gangsters, a DS full of enlisted men, Tarkin , everyone on board, etc... all which hes done.
Or even if he's upped his game adding force lightning to fight and potentially out to
kill someone like Snoke, his guards, or whatever his new Knights are.
None of that qualifies, as he's done comparable before, and stayed on the side of light.
It really becomes about motive, even if he gets all broody and intense about it, close to the brink, none of that would mean actually falling to the Dark Side.
So what are you all talking about when you say Luke goes Dark?
What I consider Dark Side is turning on the Rebels, and Killing his own family? Potentially Leai? Out of anger, killing Rey, torturing, slaughtering a bunch of innocents, a village, kids, a planet out of hate, anger, all to get his way?
I don't see it.
Is this really what you want to see or imagine for Luke? LOL!
It's not happening.
Probably the deciding moment what it will come down to is; will he kill Kylo out of anger or not, especially if Kylo killed Lukes "mate" Rey's mother, and if Kylo next has a hand in Leia's death.
If after that he still senses something redeemable in him?
So that really depends more on Kylo than Luke.
There is Rey's vision where it looks like Kylo is really the one trying to save Rey, and kills the KoR.
I can see either Luke or Rey about to kill Kylo, and the one stays the other's hand knowing there is redemption even there.