Well put, J ... and Carl. There are several ways this movie gets us not just to understand the characters' emotions, but to actually share them. This is one of them. (Another one is keeping love scenes scarce and abbreviated, so we always feel there's never enough time, too.)
Originally I was a total agnostic, thinking it could go one way the other. After reading other discussions, I'm still uncertain but, if I had to choose one, I would pick murder. There are lots of ominous signs, such as the dead sheep, pointing that way.
But that still leaves me exactly where Ennis is: leaning toward murder but not knowing for sure. And not knowing (according to a bunch of people in this very situation, whom I interviewed years ago) is more torturous even than knowing the worst.