With the “slaughtered all students” thing, don’t forget that “Snoke already turned his heart.” This isn’t the genesis of Ben Solo becoming Kylo Ren inside, it’s the final straw that triggers his external actions. So, slaughtering everyone tracks fine to me. I always interpreted “the final thing (Luke) saw were the eyes of a frightened boy whose master had failed him” as more about Luke seeing his own failure reflected in the moment (and some of Ben’s true nature/motivations deep inside) than Kylo actually literally being a little kid afraid of Luke’s scary lightsaber.