If Karma holds true (what goes around, comes around), then why do people believe in revenge and vengeance?

Karma and karma phala, i.e., the actions and reactions to the actions are true, but that does not mean that those who are not knowledgeable and pious will automatically become free from the feelings of hatred, mistrust and revenge. Some don't believe in Karma and while others believe in karma don't act according to the instructions of the scriptures. They only profess but not live up to the standard of pious actions.