Who among us hasn't stolen a few coins from the house of a struggling shopkeeper? Or driven over the odd pedestrian or two as we recklessly sped to deliver drugs to a crime boss? Or ripped the living essence out of a little girl just because we wanted to shoot even more bees out of our hands? This is what we do in games and, for all the moral context developers try to shove into the experience, our first impulse isn't to feel bad, but to wonder, "What's in it for me?" And since most games allow for unlimited retries, if we don't like the consequences for one action, we can always go back and try again.
Game strategies are largely driven by utility. In evaluating two competing paths, most gamers I know tend to view them based on cost vs. rewards. There are even entire sites dedicated to just this type of calculation. Even in World of Warcraft, which is as rich in context and lore as almost any game out there, the community's vocabulary displays a statistical bias that places function above emotional content. All the talk about Damage Per Second, Heal over Time, or 3-Minute Mages proves that the mathematical foundation of the game, which is supposed to serve the larger experience, has for many players actually become the experience.
Since most videogame characters are just a walking collection of numerical variables, it's easy for most players to overlook the emotional content of a decision and focus instead on the gameplay rewards. With that sort of perspective, you're not playing the character anymore; you're just playing his or her stats, and morality just becomes one more mechanic to exploit for maximum gain. My Commander Shepard might be a high-minded idealist, but he's not above riffling through the pockets of dead people for a few extra credits. After all, I'm trying to save the world here and, let's face it, it owes me.
In fact, seeking out those gameplay rewards is what gives me access to even greater gameplay rewards down the line. Given that system, it's tempting for players to abandon any sense of their character's ethics (which should kind of be the whole point of roleplaying) and instead make moral decisions based on what powers they want to have or what type of gameplay they like. Deus Ex's stealth options were a better reflection of the values I wanted to express through J.C. Denton, but the shooting options were more in line with what I wanted to play. The tension between the two impulses was interesting, but combat won out over character most of the time.
What's more interesting, and not often seen, is when the rewards for one type of gameplay are in direct competition with the rewards for another. It's more fun, for instance, for me to be a crook in a game like Morrowind than in Grand Theft Auto because Morrowind actively works against that impulse and requires a sacrifice from the player. Yes, crime has consequences in Grand Theft Auto, but there's little moral structure to rebel against and being bad without the option of being good isn't particularly compelling.