But you were such a good student

Why is it that the bad guy was always trained in the ways of the magical arts by the guru of the good side? I mean don't the bad guys ever teach anyone? It always seems like someone is going on and on about how the big bad guy was their best student ever and they can't believe they turned to evil. I mean now they have money, power, and fame. Wasn't living in poverty living off goodwill good enough? Some day I wanna here I have no idea who this bad guy is I wonder if they opened a bad guy school? Or even huh this isn't one of my students I wonder if it's a student from that other guru? It would be interesting.