On the subject of villains who are their own worst enemy, Dr. Jekyll and Mr. Hyde is an obvious example.

 

More complicated though is the saga of Gilgamesh, a tyrannical king who, when he met the wild man Enkidu was tamed by companionship. But upon encountering the giant Humbaba in the cedar forest, it was Enkidu who urged Gilgamesh to abandon mercy, slaying him on the forest floor in spite of his begging.