I like the use of "duty" here as letting Middle subvert expectations and what we've seen so far. The Oni transformation seems to mess with emotion, both heightening desire to fight/kill and undermining the memories and such that form emotional foundations, again with a flow towards violence. But duty is somewhat orthogonal to the normal spectrum: one can hate something or love it, find it funny or sad, noble or pathetic, and yet still feel a sense of duty towards it or something bigger it is part of. And here of course Middle's duty is still towards a dimension of combat and strength which also aligns better. So it's not like he's being a pacifist, but he's also still able to maintain his self and be discriminating towards his targets. Nice to see it from that guy.