I wonder how much guilt Ewen feels, later, over that heat-of-the-moment "Never!" that in the end isn't really a never. Lachlan's becoming a deranged vengeance-machine he would take as expected (he would do the same for Lochiel - and Nepuvr znxrf uvz cebzvfr abg gb sbe Nepuvr'f orgenlny - even Archie the doctor casually implied in the last chapter that he'd like to toss Captain Greening off a cliff!), and if it had been...literally any other Hanoverian officer Ewen's reaction would have been a resounding meh. He doesn't bear any responsibility for Keith's death, but he does bear some for Lachlan's.