A lot of prequels of classic or popular movies are showing the main villain or supporting once being a good guy. And it most ways it's the hero/main character's fault somehow.

Pan and Monsters University or two I can think of. Now Pan isn't out yet but the trailers clearly show Hook is good and a close ally of Peter's.

In Monster University (SPOILER ALERT)

Randall Boggs was a good guy at first. He was roommates with Mike. But while Mike wanted to study, Randall wanted to party with the cool kids. Sully showed to be quite a jerk throughout the movie and Randall was sometimes a victim of it. The straw that broke the camel's back was when Sully screwed Randall up during the scare competition that gave birth to the evil Randall we know from INC.