The Man in White is a character who goes around dressed entirely (or predominantly) in white wherever he is. The reasons for this are not important, but what it signifies to the audience is; and it just so happens that in most cases, it signifies that the character is up to no good.
There's just something about a man in white that weirds people out. One possible explanation for this is that since conservative men's attire has been dark-colored for the longest time (black, charcoal, and navy), looking upon a man in white creates a sensation not unlike a color-inverted image. Another one is that since white is also the color of snow and bone, an all-white ensemble evokes sensations of coldness and death.
In the US, white suits have also come to be associated with Southern plantation owners, leading to the birth of the Fat, Sweaty Southerner in a White Suit. As such, white clothing in American media can also be used to suggest pride, avarice, ambition, and questionable morality, which is why you can see so many mafiosos, drug lords, and corrupt corporate executives in such garb.
