I actually thought this worked quite well. Niko has a history of violence both with crime and war which he tries to run away from but because of Roman's debts he ends up rubbing shoulders with the Liberty City underworld and is soon pushed over the edge. An often overlooked and easily missed line of spoken dialogue comes early in the campaign where you fight a knife wielding man in a warehouse near the water. You are supposed to knock this guy out and take his knife and leave him. Should you knock him through the window into the water, Niko will regretfully exclaim that he promised himself he wouldn't kill anyone in this country.
At which point (probably on the way there alone, the way my brother drives, lol) you'll have killed enough people already that even in countries without the death-penalty they'd hang you for crimes against humanity...
I see where you're coming from, and the difference is subtle but there, and it's all in the character:
In RDR at least Marston constantly acknowledges he is a bad man and the way I saw his character is that he wasn't done with the violent life he had lead because he thought it was morally reprehensible! In fact he defends his actions on multiple occasions!
In his screwed up little rule-set he did right! The only sin he actually seems to have a problem with is betraying his wife and whoring around.
Mentally, he is a child. Fell for the first woman who was interested in him, even though he knew she was a whore (not my words) and even though she's kind of an ***, and he feels great for doing anything for her, although he really does it to feel better about himself.
This misplaced loyalty is what drives him and it's the only moral rule he has (and interestingly, that's the
one sin the game
won't let you commit! You can't visit whores)!
Other than that though, he's a cold-blooded killer and blasting a man to bits doesn't seem to bother him more than pulling the wings of a fly would bother a normal person.
So, while I played an "honorable" John Marston, because ridiculous rampages are just that, ridiculous, I really don't think the occasional robbery or dragging someone behind your horse would bother the character a whole lot.
Niko turns out similar later on in the game. He's also a tragic hero who wants to leave his violent past behind but falls back into the same patterns (except they're more psychotic now because he isn't in a war-torn environment of pure violence anymore but a civilized metropolis).
He wanted to quit violence for the sake of quitting violence though! Not just for the sake of staying out of trouble!
The story of how he is coerced, pushed, forced and driven to become a stone cold killer once more is cool, but as a player, and this is the problem I was referring to earlier, you can be a psychotic monster way before it should happen to the character.