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For me, easily Spec Ops: The Line.
That game took an incredibly dark turn during the White Phosphorus scene. You know those top-down camera views in Call of Duty where you shoot enemies from the sky? This game does that, too. And then, boom, you find out that you just burned a refugee camp to the ground with phosphorus bombs. And this game actually forces you to walk through the travesty you just caused, and see all of the flaiming remains of soldiers screaming for help and buildings and cars burning and on fire. But, the most haunting part of that scene is the burning carcass of a woman holding her child as the white phosphorus breaches through the gate.
That image has been burned into my brain. 10/10 will not be playing again anytime soon