How Do Characters Go Bad

Every story needs a bad guy to make the hero shine. WithoutDracula, Johnathan Harker would have just been a wimpy solicitor. LukeSkywalker might still have become a hero as a squad leader in Star Wars, but, withthe help of a shove from daddy Darth he became a more awesome and memorablecharacter as a Jedi Knight.
The villain and their background has everything to do withhow heroic a hero will turn out to but what's the recipe for turning acharacter to their dark side?
One cup of attitude, two ...