Why the ending makes sense....spoilers...
The ending is not a way to excuse the evil behavior of the Robert Ryan character in any way. It doesn't even make a moral statement against bounty hunting. The ending is the way the Jimmy Stewart character is able to let go of the past injustices done against him and his bitterness and obsessions that have resulted.
It's simply a way to move on and get a fresh start. Doesn't hurt that he has a woman there to walk with him in life, that certainly helps and he did kill the bad guy so that also helps him move on.
A more intelligent film than most people give it credit for and there is nothing old or dated about this film at all they could make it again today.
There is a Randolph Scott-Budd Boetticher film called "Ride Lonesome" that also deals with some of these same issues and builds up in some of those same ways but that is a personal vendetta for a horrible wrong done to Randolph Scott's character not a general bitterness that the Stewart character has to live with.
People need to give films like this a chance because there is a lot more there than you might think after one viewing.
8.5