Future<void>

That's a hard question and I'm coming up short. Prototype and Carrion establish clear motivations and end goals for their monstrous protagonists, and don't evoke an attempt at the same kind of reaction of empathizing with nihilism that Ultrakill does.

On the other end, the original F.E.A.R. and its spiritual successor Trepang2 do the depersonalization well. The protagonists in both are deemphasized and put the players in their shoes through intense first-person gameplay, the story happens around them but they barely influence those events beyond when those events get in the way of their rote purpose¹. I would say they both fall short though because in both cases the protagonist is present because of someone else's deliberate decision, dampening the "war without reason" aspect because the reason still exists with a degree of removal.

1. The optional missions in Trepang2's campaign can be an argument against this, but it still feels less like proactive decisions from the protagonist and more that other characters know that their violence will conform to the nature of any given mission in the same way that liquid takes the shape of whatever container it is poured into.