Two of the greatest experiences in fiction that I have ever had were pretty dark.
In Berserk, during the Eclipse, when the Band of the Hawk risks everything to save their former leader who was being held captive by the mad king… Only for him to betray them to the sinister god-like entities who would restore his body in trade for their lives. Almost every character dies there, and the protagonist’s love interest is raped while he cuts his own arm off to try to save her. It’s a very powerful, desolate and emotional scene.
And in Gantz, when everyone dies in a mission but the protagonist. He goes back to his normal life and you think things can’t get any worse, but before you or the character have time to even to digest what happened, there is another mission. He is sent to face alone a group of aliens that he cannot defeat. I remember how desperate that situation was like it was yesterday.
I think that, overall, the power to evoke emotion on others is the greatest strength of any medium. Feelings are what stick with you even long after you have first experienced them.
Concerning a game, I think that this can be even more powerful, as what you experience is(or should be, at least here) a consequence of your own actions. And that’s the most important thing, I believe; to give players meaning, context and consequence, instead of just giving them the option to mass murder everyone because why the fuck not.
There are plenty of GTA-like games that let the player cause mayhem, kill civilians, do all kinds of crazy shit. They might be fun to someone on the short run, but who really gives a shit about it? There’s no emotional weight, there is no moral dilemma, nothing of the sorts. Just someone randomly killing NPCs.
Exploring the consequences of devastating acts is fun. The books of Broken Empire trilogy do that very well, for example. The does all sort of horrible things in many levels, from nuking a town to killing a baby. He is not always in his right mind, as there are people influencing his actions sometimes, but he takes the blame regardless, as you see how haunted he is by those things even when he justifies them to himself.
So am all in for dark stuff, as long as it isn’t gratuitous, but that’s just my personal taste. I think the player, given proper context, should be granted the choice to act as he feels he must, but there must always be some kind of payback. If you want to act as a “villain”, there should be someone ready to stand against you at some point, someone looking for vengeance from your past acts or people rebelling against you. There must be some kind of reaction provoked by your deeds.
That being said, I personally agree that there must be a limit somewhere on what the player should be able to do. A world can be dark enough without letting the player be the perpetrator of all the possible darkness in it.