I must respectfully disagree with you. There was nothing sudden about it. Actually, I kinda knew something like that would happen. Throughout the whole game, Vergil seemed to be really eager about overthrowing Mundus. Also, the way how he treated Kat as nothing but means to complete his goals (another way to say item), really made it obvious he had something sinister in mind. Besides, I knew NT would use the sibling rivalry to make the game more interesting; it was unavoidable. The first moment I saw Vergil, I immediately thought to myself "Oh, he's gonna be a boss!"
Maybe NT didn't make the ideal DMC (they did according to me, but I am talking about the hate bandwagon riders), but one thing it did right is that it worked with clues very well.
Edit: The title is very misleading by the way.