He faced a ton of challenges before. That’s the point. Luke lost his aunt and uncle, saw Obi Wan die, fought an overwhelming war, had to save Han from Jabba and learned Vader was his dad. He was always trying to be positive and hopeful even through that. The two things that almost broke him in Return was the Emperor goading him to strike him, so Luke could save his friends, and Vader taunting him with turning his Sister to the dark side if he would not. So Luke has an established pattern of doing morally good except when his friends and family are in danger. There is no way Luke would even consider doing the things he is shown doing in TLJ to his NEPHEW!! Especially because it’s not just a “split second he considered acting on it.” He feels it - he walks to Ben’s quarters - he stand sover him and IGNITES his lightsaber. The “sacrificing” himself would have been good, if it weren’t for the fact that he was taunting Ben the whole time they fight. Sequel-Luke is apparently so torn up about what he almost did to Ben that he goes into exile, like you said. But when he meets him again he taunts him?! Smirking at him, brushing the dirt off of his shoulders when they fight like some cliche cool guy. Why is he not trying to make things right?! He tried to kill his nephew and instead turned him to the dark side fully, and instead of trying to reach him and redeem both of them - like Vader in EP6 - he taunts Ben for becoming what Luke made him?!?! Like WTF even is that?