I'm currently writing a character in an original fiction of mine who is in quite a similar situation to Marx and I'm trying to devise plausible reasons for his loyalty, so this question is quite interesting to me. What were you thinking about to "make it work?" To me, Marx has some personal attachments to Garon being his son and remembering him for the kind father he used to be. In a lot of ways I think Marx behaves similarly to the child of an alcoholic or abusive parent who despite the terrible behavior of their parent feels a sense of misplaced/confused loyalty and a need to "protect" or "save" the parent.
Also, if Nohr is in such disastrous straits Marx may feel Garon's brutality is justified in order to save the country; he sees it as disturbing, but pragmatic. A necessary evil so to speak.
I'm interested in hearing other reasons as well.