In ICONS Superpowered Roleplaying: The Assembled Edition (page 187), it's said that a villain's motivations help define their goals and plans and establish the villain in the game setting. For those Game Masters with problems devising motivations for their impromptu villains, here it is a list to help you out. As always, please feel free to discuss, critique and evaluate in the comments below.
Roll 2D6 or pick up the one(s) you deem appropriate.
2 Insanity: This villain is insane. Roll again, twisting the result to make it very extreme and deeply irrational. (For example, if you re-roll and get "Carnage", then perhaps the villain wants to destroy the entire world).
3 Vengeance: Something was done to this villain that they think was unjustified. The villain wants to pay back the ones he thinks are responsible.
4 Dystopian: The villain's world-view drives them to establish an evil, oppressive government, religion, or other authoritarian institution.
5 Thrill-Seeker: The villain is mainly driven by the thrill of accomplishing daring crimes.
6 Greed: The villain is overwhelmed by desire for wealth. Even when great wealth is achieved, more must be gained.
7 Anarchist: The villain enjoys inflicting harm on authoritarian institutions.
8 Prejudice: This villain hates and despises some racial or ethnic group, wishing to undo their accomplishments and subjugate them to his/her will.
9 For Hire: The villain has no particular motivation of his or her own. But they've got a reputation for being good at crime, leading other criminals to employ them.
10 Egotist: The villain suffers from the delusion that she has a grand destiny, and deserving of wealth and power.
11 Publicity Seeker: The villain loves to be talked about, to have his picture in the paper, to collect newspaper clippings and watch TV coverage of his own crimes.
12 Carnage: The villain loves to destroy things, preferably ancient, famous, or venerated things.