Although he had an insanity excuse for it, Hercules, the greatest of the Greek heroes, murdered his wife and children, and killed a couple of his brother's kids, as well. It was to atone for this crime that he started on the twelve labors. Read about why he went insane and who drove him to it:
Hercules - Madness and Atonement

Labor One - Nemean Lion
Labor Two - Lernean Hydra
Labor Three - Cerynitian Hind
Labor Four - Erymanthian Boar
Labor Five - Stables of Augeas
Labor Six - Stymphalian Birds
Labor Seven - Cretan Bulls
Labor Eight - Mares of Diomedes
Labor Nine - Belt of Hippolyte More on Hippolyte
Labor Ten - Cattle of Geryon
Labor Eleven - Apples of Hesperides More on Hesperides
Labor Twelve - Hound of Hades