It's hard to make a good RPG story without a near-universal antagonist. And while good and evil are obviously subjective, an antagonist that is very compelling for many different types of characters (like the Master or the Enclave) is the closest you'll get to a "bad guy." Torment did. Stalker didn't. To be honest, I don't even remember my character's motivation for going through the plot in Stalker. I was just trying to beat the game. And both of those games had much more railroading than I'd like to see in a FO game.