I was going to guess that it's Armond in the casket. And say that Shane ultimately blames Armond for Rachel leaving him, as the botched room is what initially caused the rift between Shane and Rachel, they would have been perfectly happy on their honeymoon otherwise, etc, etc. Armond is high as a kite, there's an altercation, and Armond dies, but it's inadvertent, and Shane ultimately isn't held responsible. So when Shane is staring out the terminal window in the flash forward, he's staring at his nemesis in the casket, wondering how it came to this. But then I quickly realized that if Armond lives in Hawaii, his body wouldn't be on a plane, so I'm back to having absolutely no clue.