This is my personal opinion on this game and how it ends. The game’s lead writer Walt Williams has said that although it is open to interpretation, he believes Walker and his team died in the helicopter crash, and everything after that is Walker’s purgatory.
To extend this, I think that he is stuck in a loop; there are a few minor clues that seem to confirm this, such as before the white phosphorus mortar strike when Lugo says, “There’s always a choice,” and Walker responds with, “There’s not.” This implies to me that Walker has no control over his actions; he is being forced to relive the personal hell of his final moments over and over. Later, when you reach the stage of the escape from Radioman’s HQ in the helicopter, which was also featured at the beginning of the game, Walker bizarrely states, “Wait, this isn’t right, we’ve done this already!” This again implies that Walker has done this before, maybe hundreds of times, and is only faintly aware of it.
Then we reach the end where he confronts Konrad and is offered the option to kill himself, thus taking responsibility for his actions and breaking the cycle of his personal hell or continuing to delude himself and blaming Konrad, continuing with the cycle indefinitely. Walker is a ticking time bomb from the start, despite clear orders to locate survivors and leave he insists on rescuing the locals and finding Konrad, it is implied that when Konrad rescued him in Kabul, they suffered horrific circumstances which left both men with PTSD that ultimately affected their judgment later on.
Walker now has a strange obsession with Konrad, which makes him see visions of his face on billboards, something we don’t realize on our first playthrough as we don’t know what Konrad looks like. The white phosphorus incident seems to have been the trigger that snapped Walker’s mind completely; however, after this, he began hallucinating vividly and regularly, both visually and audibly, and he made Konrad a scapegoat for his own mistakes. This rubs off on his squadmates who, while terrified of the circumstances they find themselves in, are even more terrified of Walker, who insists on pushing forward with psychotic single-mindedness despite their reservations about his sanity. I think it’s interesting that they didn’t try to kill him, maybe out of fear they would take the blame for his actions? That they wouldn’t survive without his leadership? Or out of pure, misguided loyalty? Who knows? Of course, the game can also be taken entirely at face value, that this is Walker’s only time through Dubai and the deja vu in the chopper is simply part of the mental degradation he is suffering.
The different available interpretations are what makes this game so great. I especially love the ending, so original and the voice actors deserve enormous credit for the effort they put into bringing these characters to life.