If you liked tombstone you should like Quigley.
If you like dark westerns, get The Unforgiven. (Not the movie of the same name from the 50s, but the one starting and directed by Clint Eastwood.)
Tombstone was probably as dark as I can take it. My favorites are comedies and fast-paced action. 1600 PA Ave; Reds and Reds 2; Jumping Jack Flash; Love of the Game; Our Man Flint; Bells of St Mary; Laura, Iron Man etc When I used to write, I wrote romantic comedies.