I probably wouldn't want this game to actually exist, but it's been stuck in my head for years so here goes. I described this one a while ago. A friend of mine was on mushrooms once and described a first person WW1 game where you're an Austro-Hungarian courier running across battlefields. There would be parkour, time management, stealth, stuff like that. Sneaking through trenches and whatever. At first the missions go ok, easy enough. But then you're given more complex missions that waste your time, or are foolishly planned.

Your character begins mumbling under their breath about how the generals are doing everything wrong, the war is lost. Your character becomes more deranged as the missions become more fruitless. Eventually your guy will start screaming deranged conspiracies and wild racist shit. There would be a mechanic where you start to need amphetamines to function.

Then in the last mission you catch sight of your reflection in a puddle and you've been playing as Hitler this whole time.