http://www.youtube.com/watch?v=o_GWeu4lU2M
Read the top comment.
[spoiler]So Walker actually never went home. He either killed all those soldiers, died trying, or shot himself when he finally found out the truth. A pretty depressing ending to an already depressing game. And I thought that he actually had a chance of redemption.[/spoiler]
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Secret fifth ending: [spoiler]You don't play the game, therefore never comitting the acts that would damn you. Unless of course Walker died in the helicopter crash at the beginning and this is all his dying dream as he relives the last few days in his life. [/spoiler]