Essay help- what do you call when someone kills purely out of evil, where there is no real motivation?

I am doing a comparison essay on "killings" by Andre Dubus Vs "Cask of Amontillado" by Poe. The revenge in the first story is out of love. But the Cask revenge is purely eveil merely to avenge the decline of family's lost pride. Now can someone help me with a word that can summerize the later killing. can it be called RETRIBUTION? Real help appreciated.