I don't know if you could actually call this immoral (probably just amoral, now that I think about it), but whatever.
The most immoral thing I ever decided to do in a game was to decide to play Prototype all the way through, because Alex Mercer is just about the most horrible video game character in existence. You really don't have any option but to murder and eat people for health, your actions inevitably wind up costing millions of innocent people their lives, and you'll have to kill just about every single marine in the entire US armed forces by the end of the game, not to mention the billions in property damage.
Of course, it's probably better to call Alex Mercer amoral rather than immoral because
The most immoral thing I ever decided to do in a game was to decide to play Prototype all the way through, because Alex Mercer is just about the most horrible video game character in existence. You really don't have any option but to murder and eat people for health, your actions inevitably wind up costing millions of innocent people their lives, and you'll have to kill just about every single marine in the entire US armed forces by the end of the game, not to mention the billions in property damage.
Of course, it's probably better to call Alex Mercer amoral rather than immoral because
about 3/4 of the way through the game Alex Mercer realizes that he's not really human anymore, but is, and has been since the game began, a sentient virus taking human form. Calling Alex Mercer's actions immoral loses some of its edge when you realize that the loss his humanity has removed any reason Alex would have had to be moral. Of course, it turns out Alex was the one who unleashed the virus on Manhattan in the first place, trying to save his own skin no less, so he was a bastard even before the game started.