Another way to slice it is that for an 18th century gentleman, personal honour was paramount, and more important than military duty. If Keith's honour would be better served by saving Ewen, which it might be, since Ewen let him go in Edinburgh, then he might let Ewen go...of course, Keith would suspect this line of thinking as probably being influenced by his emotions.

Then again, I can't pretend that I understand their sense of honour; it's really very alien to me. I keep coming back to this Duffy quote, which expresses both aspects: Having encountered so much that was bizarre or selfish about the cult of honour, it is impossible for us to withhold our admiration for the importance that the 18th century attached to the primacy of the individual conscience [in potentially refusing to follow orders].