That happens across several different scenes of the movie, so not an "isolated" sin, but a very thorough trashing. There's the green milk, the total 180 in his personality for no reason, the attempt to kill his sleeping nephew and probably other things I've forgotten.

While the scene where he fights Kylo looks cool, that happening because "it was a force ghost/projection all along" felt like a litmus test of how much bullshit people were willing to swallow. Sure, the force is basically space magic, but physically clashing sabers one minute and being completely holographic the next? Funnily enough, if the fight was entirely Luke dodging Kylo, that would've worked so much better.