I've begun to wonder where Nintendo is taking Luigi. He's becoming rather enigmatic.
In Super Paper Mario, he's one of the "key ingredients" for the final boss. In Mario & Luigi: Partners In Time, Luigi alone could not enter a star door because "there was darkness in his heart".
And even recently in Super Smash Bros. Brawl, his Final Smash is, according to his character trophy, "A reflection of the darkness he embraced in his brother's shadow". Also in Brawl, a secret taunt by Snake about Luigi causes the Colonel to go mad.
Nintendo has been building up on this "Dark Luigi" aspect for quite some time now. Examples go so far back to the original Paper Mario, where Luigi eats a snack intended for Mario. So I've just got to wonder where they're taking Mario's beloved brother.