I agree re the ending. To me, it isn’t so much about McDonagh trying to be clever and NOT give the audience a ‘conventional’ happy ending, but rather grounded in the story’s key characters combined with the morals, if you will, of the story universe he explores. Every single character exists in the realm of Disunity, each has their own flaws, wounds, and embattled psyches (perhaps with the exception of James who is a deeply giving soul). Therefore, if McDonagh were to wrap up things with neat bow — “Here’s the killer!” — that would have felt false to me.
The ending is much more nuanced reflecting the complex nature of human existence. Indeed, one could argue the mixture of drama and humor which runs throughout the story reflects this very same point, they can exist at the same time, as uncomfortable as that may feel for a script reader or movie viewer.
This may be the most Coenesque movie McDonagh has done exploring the dynamism of violence, fate as a ‘drunken driver’, themes which appear in Coen brothers’ movies over and over. So does unresolved endings. Mildred and Dixon head off to either kill or maybe not kill a guy who may or may not be a rapist… FADE OUT… again, this reflects the complex and ambiguous nature of the human experience.
BTW my point in comparing Three Billboards to the Coens is meant as a compliment as I hold the Coens in the highest regard — master storytellers.