Could Matthew Garth Kill Dunston?
Everyone has a problem with the ending. Dunston's 180 change of character doesn't ring true. I suspect the director and writer also had a problem.
Dunston would have had no problem killing Matthew, given his descent into near madness. I just don't see Matthew killing the father he loved. I also don't see Tess killing Dunston because then there's no way for Matthew to live with her afterwards.
Possible endings:
Dunston kills Matthew, then dies of his wounds; A true Shakespearean ending!
A wounded Cherry staggers back into the scene and kills Dunston; most likely ending.
Dunston kills Matthew, recovers from his wounds and persuades Tess to have his child.
Dunston and Matthew do not kill each other, but Tess decides to ride off with Cherry. (Actually, I think this is what happened offscreen).