I see a lot of people criticizing the WP scene, and I get why... but I think an element of the railroading with that is this is the first moment Walker's actions are ripped out of your hands. His squad doesn't want to do this. You, if you know what is coming or know how horrific WP can be, don't want to do this. Walker doesn't care. This is HIS mission... as he sees it at that stage. He's already off in the head, obsessing over seeing this through and being heroic. Yes, on the most obvious level, he COULD walk away, report to his superiors. He won't. That isn't who he is. He plunges ahead, even knowing deep down this isn't what he's here for, isn't what he should do... because he wants to see this through, and he's dragging you along for the ride.
I would argue that it's one of the reasons most of the choices in the game come AFTER this scene... because his mental break and 'Konrad' test, all that, is his brain DESPERATELY trying to remind him he DOES have a choice. But other than minor elements here and there, he's not listening. He's lost in his delusion. He's the hero. He's going to right the wrongs. Take out every bad thing he's ever done on 'the bad guys'. He isn't so much a mirror on the player, though his efforts and the climax do stare you in the face to ask you why YOU played this far. He's the worst elements of the (no pun intended) escapist, retreating into a fantasy to blame his actions on. He's everything most of us hate in a gamer, made larger than life, and then we're given the controls and told to play as him. And while by the end I had more pity for him than anything, I still deeply disliked him, which is kind of the point.
Konrad's famed big speech is aimed at Walker but hits you too, IF you were playing through the game gung-ho Rambo super-hero mode heedless of what you've done or who you've hurt. But if you tried your best to corral Walker, to force him not to lose himself as a monster, to not play the game, spare lives, etc., the only one that speech is for is him. Yes, you could have turned the game off any time, but that's not all the game is asking you to consider. It's asking you what YOU chose when it DID give you a choice. That's why in the ending what you see is based on what you decided to do in those moments.
To sum up, walking away was an option only in the abstract; in the diseased and obsessed mind of Walker, already just short of cracking wide open into full-blown insanity, there was no walking away, no retreat. Heroes don't walk away. And if that sentiment isn't your cup of tea, if it in fact disgusts you especially considering what came after, congratulations. You're not Walker. You didn't cross The Line.