Every time I played that boardgame Clue with my little brother, due to some fluke of chance, Colonel Mustard always murdered somebody in the Library with the Candlestick.
I think that's why that game never got more popular, we tried playing it like four times, and every time we did, it was always good ole Colonel Mustard in the Library with the Candlestick.
Sorta takes all the fun out of solving the mystery when its always the same guy killing people in the same place with the same murder weapon.
I dunno, I guess our game was broken or something.
But as far as motivations go, it probably had something to do with him being a military guy represented by the color Yellow.