Revenge

See to it that no bitter root grows up to cause trouble and define many - Hebrew 12: 15

There isn't a doubt that revenge fills the heart with bitterness and resentment. It injures the body and soul. We need to deal with our desires for revenge before we turn into vengeful people. It drains our personality. It can even contribute to mental illnesses. 

We are not created for anger and rage; we are created for love. God built us to love and we go against His design when we decide to hate.

When we have that urge to 'get even' it more often than not only disturbs us - the person who has offended us is unaffected.