I've never had to deal with players veering "left" or "right" so to speak (yet), but I'd definitely go along the line's of Fallout 3. Basically, certain factions (Talon Company and the Regulators), will begin hunting the player if they swing their 'Karma' to far in either side. Talon Company is hired by the big bads to finish off your goody two shoes character, while the Regulators are contracted to bring your tyrannical murder-machine down.
My players (so far), have never had a problem with falling off-topic. Having a good story that keeps them interested and having some sort of mystery just beyond their grasp will keep them enthralled.
I also play the role of the "Cliffhanger GM" to always keep them coming back