Metal Gear Rising: Revengeance/Nightmare Fuel

Jack. The. Ripper. Raiden said this side of himself was a remorseless murderer who didn't give a shit about causing bloodshed and was a demon he fought hard to bury since MGS2. When he decides to let his inner demon out, it was just as pants shittingly terrifying as he claimed. Hell, even Monsoon was horribly creeped out at the change in Raiden, and he's the one who attempted to goad it into happening in the first place.
- Further ramping up the Nightmare Fuel is one of Monsoon's potential death quotes at the end of the ensuing boss fight. Sometimes, he maintains his nihilistic "nothing really matters" attitude even when he's being hacked to bits. Other times, this normally unflappable, defiant sociopath completely loses his shit and begs for mercy.

Sundowner may be a goofy, entertaining Fountain of Memes, but he's a depraved monster of a man, and the game isn't afraid to remind you of that fact. When you confront him just before his boss fight, you do so in a server room full of child brains that have been removed from their hosts. And are still alive and aware of what's going on around them. Sundowner forces them to participate in realistic VR training that will twist them into child soldiers, and then sell the brains on the black market for fun and profit. It's not hard to shudder when you see him leering down at a brain casing in the palm of his hand, with the poor thing squirming around in obvious terror.