Red Dead Redemption Explained: What Does The Ending Really Mean?
2. We All Have To Pay For Our Sins
Marston used to be a criminal, but is no longer.
Still, in his quest to eliminate Dutch's gang in its entirety, Ross still thinks of him as an outlaw and kills, despite the promise that he would leave him alone should Marston eliminate all of his former associates.
His body is found by his wife and 16 year old son, who three years later hunts down Ross and also makes him pay for his former life - it's telling that Ross is now retired, just like Marston was.
This is of no importance to young Marston, just as it was of no importance to Ross three years earlier - his former life didn't allow for him to die in a different way than by the gun.