react on this: "I believe in karma but I practice revenge."

You want a response related to Karma, well here goes- It's your karma of a previous life time making you vengeful, or you are adding to you bad karma's by being vengeful and this will affect your path tho salvation. Frankly you can try to believe in Karma, but believing means understanding and practising. It's like people who say they believe in religion and God but yet commit sins.