I'd say such scenes have to be crafted with utmost care in order to do justice to the scene, and make it more than simply pandering for a rating. Or simply be a "because we can" scene.
The Resevoir Dogs scene is one. I'd probably quibble with Oberyn's death scene though. I'd have to go back and reread the scene, but I think it was even more brutal than how GRRM portrayed it. I'd probably put the Red Wedding scene as a better example.
That's also why I think it is a mistake to try to outline an "objective" reason for why we need M-rating. I don't think there are objective reasons for that- there are only subjective ones. If somebody says the ear-cutting scene didn't do it for them, there's nothing I can offer as a convincing counter. But I don't think that objective reasons are needed to justify wanting an M-rating. It's just a case of everyone advocating for what they want the most. In this case however, it's just that those being comfortable with an M-rating are bit more plentiful, at least as this forum goes.
Out of curiosity, what's the "most despicable moral choice" you find in it? There's certainly a lot of moral greyness in that game, but I don't recall any choice that was as "fist through screen" despicable as what ME3 provided.
Well, the KotOR series allows the playing of a Sadist, which is very questionable. The scene I was thinking of involves the sacrifice of Visas Marr. Morally grey requires there be some redeeming act to what might be considered controversial decision-making. That entire sequence goes a bit beyond that.
The game allows you to romantically manipulate someone into committing suicide, simply to weaken your enemy. That alone bumps us into morally grey. But then, they take it a step farther by giving the PC the ability to rub it in Visas' face as she's dying in front of you that the only reason you ever involved yourself with her was to treat her as a pawn. If our concern is mature content, I find that much more dangerous than an F-bomb or set of breasts.
Note that, as bad as the endings to ME3 might have been, there was no other option in that context. Those decisions are morally grey in the context of "pick your poison". There's no sadism behind the ME3 decisions, on the part of Shepard. In general, that could be said for most of KotOR's decisions being worse than Mass Effect, since they're all about sadism.