Look at how he handles rejection.
He has personally killed or ordered the deaths of so many people that its ridiculous if he is allowed to join the heroes with no consequence (and his feeling sad and wiping an occasional tear for the rest of his days while leading an otherwise happy life does not count).
Luke's students who refused to turn to the dark side (some of them look quite young), Lor San Tekka, the people of Tuanul (including women and the elderly), the people of Tehar (including children and ordering the pursuit of two children who managed to get away), his own father, a whole hangar filled with resistance members on the Raddus, and most of the surviving resistance on Crait.
And if there is any doubt that he participated in the killing of civilians of all ages, the children from Tehar mention that Kylo Ren destroyed their people and is hunting them down. Not to mention, Kaz finding a Tooka doll in a blaster mark ridden empty settlement (hinted to be Tehar), indicating children were also killed there.
This is not counting the people he has injured, tortured ad intimidated like Finn, Poe, Del Meeko, and even Rey. And if we include the bad guys, there are Hux, Mitaka, and Tierny as well.
If he had shown signs of actual remorse in TLJ, redemption would have made sense. I don't doubt his redemption but IMO its only going to happen because he is a Skywalker, and for the sake of his parents (despite him murdering his father and making his poor mother a widow). But if they throw in a romance with Rey, it would be a step too far.
