Ultrakill manages this very well.
V1 has no reason to fight other than having been made to fight and fuel itself with blood. How do you make a player emphasize with such a character—barely a character at all, "a mere object", a weapon? Turns out, what you do is design a combat system so batshit insane that it makes the player feel like it is them who is fueled with blood and programmed to kill.
The 'war without reason' trope is certainly found in villains, but has anyone else implemented it in a protagonist? Not just the motivation that is here (or at least implied) only to serve as window dressing and is practically irrelevant (see: most retro FPS), but the deliberate absence of it?