Plague

My younger siblings introduced me to Plague, which is totally addictive and fun, even if you feel a morbid sense of disgust with yourself when done, for the zeal with which you have just killed off the entire human race.  

My favorite is watching the last dwindling numbers of the "infected" population as they get killed off and having the game tell me that the last humans are hiding in holes, awaiting their terrible, inevitable fate.  I think I will not be a benevolent god.