He died so he could be a big hollywood star again. Just look at all his recent movies.
Actually, the decision
wasn't Monaghan's. The producers decided to kill him off because they didn't think there was much more they could do with the character.
i like that, in the linked article, you can actually see they're sorta making up the story as they go along. only pieces, of course, but pretty significant ones. with a show like this, i always wondered how much was planned and already decided and how much they're just like "aww... fuck it".
still, i don't understand why the guy couldn't... y'know... just not be in the water room.