so the night troll spouse possibly moved into the civ and pick up the ethics of the civ thus losing any drive to just kill folks for being a dangerous beast(and probably stayed there long after the fall of the civ if I throw out a hunch). due to how historical figures settle in to sites this is several mechanics working together to make a friendly night troll that lives in the sewers.
also I feel like Normal and DF don't really mix the game is surreal and most stories that come from it are from folks finding these interactions.
I had the experience of figuring out how to get a night troll or hostile sapient unit become non-hostile by convincing them to join a civ site mostly a player fort and just migrate themselves to the location in fort mode.
so I can see how if history had the night troll live in a site they probably settled down in the site and become part of the civ in a way pick up the ethics and upon loading the map the game went ok this unit is part of this site thus part of this civ so they will follow the civil ethics the site had.
there also another chance you talk to them in an angle where they couldn't really see you and just having a nice convo with a voice they hear.
any way night trolls are sapient creatures so I don't consider them mindlessly hostile.