Actually I had my own idea along those lines, although it's conceptuated as a game, a prequel to RDR: Red Dead Rebellion.
Basically, it's set during the American Civil War, following the actions of Bill Marston, the first member of the Marston family to be born in America.
You can work for either the Union or Confederate government (you're choice), going behind enemy lines and destroying supplies, creating dissent, freeing POWs, all that kind of stuff.
Then, after the end of the war, you'll be sent into the South as a peacekeeper if you chose Union, or, if Confederate, you are sent into the North to create dissent.
In the end, the Union government (if you chose them) throws you away like an old phone book and out of anger you become an outlaw, robbing banks and trains, and generally being an awesome Western criminal. If however you areConfederate, you join the cavalry and commit genocide on Native Americans.
In essence, it would be the first modern videogame in which you are predestined to be evil.