One thing i don't see mentioned about the game, but felt was lacking, is the fact that the spiral of delusion didn't exactly 'hit' the conclusion needed to give the player real reason to doubt Walker's perception.
Everything, including the last final battle, was still grounded very much in reality. This didn't set up the delusional ending as good as it could because the switch from 'real' to 'delusion' was too fast. All that separated the 'reality' was that little cutscene of us going up the road and greeting, the what i then thought to be real, remnants of the battalion. Given the last 5 minutes of the game, we had no reason, prior to the ending 'cutscene' to doubt Walker's perception of reality.
If after the final battle/bunker explosion, we would have had a short gameplay moment, with Walker reaching the tip of his madness, i think that would have settled in the feeling a lot better. Let me walk you through it.
Imagine just watching the sequence after the bunker, you reach that bridge and realise that there's a final wave of enemies which you have to go through. The character then picks up two LMG's or one of the stationary Gatling sentries in the game, something ridiculous, and then advances on the enemies. The player can only reasonably go forward, the screen gets red as it usually does when you are near death, but you don't die.. hundreds of soldiers falling to your left and to your right as you wade through them effortlessly, trucks exploding for the pure visceral pleasure of carnage. You are a god amongst men, a god of war.
What would that sequence achieve? Make you question what you're seeing, set up the realisation that Walker really fell down the deep end.