Lots of stories show magicians and wizards turning to the dark in their quest for power. They turn bad in incremental steps as their moral compass shifts more and more askew. Roleplaying games have modelled this in different ways.
A recent thread at BRP Central discussed magical corruption and I put a couple of ideas forward. Here are my house rules for the BRP family of games, now that I’ve had more of a chance to think on it some more.