The Creature is Mad!

"'You can blast my other passions, but revenge remains -- revenge, henceforth dearer than light of food! I may die, but first you, my tyrant and tormentor, shall curse the sun that gazes on your misery'" (Shelley, Chapter 20, pg. 123).

Frankenstein decides to not create a female companion for the creature. The creature shows a gross lack of morals and is set on fulfilling his threats of revenge - he does this through killing the loved ones of Victory. He must long for that same love, and being denied it personally from his creator, and from being denied a companion, this is probably why he chooses to kill those dear to Victor, so he can feel the same pain. The creature finds its purpose in destroying good because Victor did not allow him any happiness.