Why does Moses seem to take Khizr slaying a boy lightly?

Khizr reluctantly allowed Moses to follow him, stating Moses wouldn't be able to bear with him. Along their path, from Quran 18:74-76: > And so the two went on, till, when they met a young man, [the sage] slew him - [whereupon Moses] exclaimed: "Hast thou slain an innocent human being without [his having taken] another man's life? Indeed, thou hast done a terrible thing!" > > He replied: "Did I not tell thee that thou wilt never be able to have patience with me?" > > Said [Moses]: "If, after this, I should ever question thee, keep me not in thy company: [for by] now thou hast heard enough excuses from me." [Spoiler alert.] We later learn that Khizr was commanded by God to slay the boy who will be replaced by another, believing son; he explained this to Moses. (See also: https://islam.stackexchange.com/q/2464/17163) However, when the slaying took place, Moses was not aware that Khizr was commanded by God, yet somehow accepted Khizr's non-explanation as reason to not take action in response to, what was from Moses's perspective, murder. *Question*: Why does Moses take the slaying of this boy so lightly?