My use of "grim dark" is in response to those I've seen comparing him to the likes of the Punisher (not here in other threads). Or a ruthless killer without remorse, because of their interpretation his opening scene.
Yet I disagree with that "grim dark" assessment of the character.
To the contrary, although this is a character who goes to a dark place, that is in fact because he feels remorse.
That is "the cage he carries with him" that Chirrut speaks of.
We instantly see the heavy weight of that kill, on him, and for the rest of the movie.
His series will likley explore how he gets to that point where he can pull the trigger.
Again I think the Bourne movies will be more than template.