I hate to interrupt this discussion, but I once read this on Wikipedia (this is a direct quote): "Due to the fact that, in the game, Madame Clairvoya claims that Bowser has been 'soundly defeated' by Mario, many believe that Luigi's Mansion is set after all the other Mario games, even those yet to be released.", implying that LM took place after every other Mario game; in between that time, Mario killed Bowser. It's not fully explained if this true, but it's an interesting theory. It may be possible that Mario did kill Bowser, but he was revived shortly after the events of LM, similar to what happened in NSMB. Another theory of mine is, indeed, LM takes place after every Mario game, and Bowser truly is dead.
Also, perhaps "Soundly defeated" doesn't necessarily mean killed; maybe Mario knocked Bowser into a temporary coma or maybe Bowser broke an arm or something.
What do you guys think on the subject?