Although we all make mistakes in life, some persons on earth feel that any person who intentionally or unintentionally harm somebody should be made to suffer the consequences of their wrongdoing. Peaceful people everywhere has learnt to deal with disappointing calamities in life by forgiving the person whom was responsible for it and moving on with their lives.
Unluckily, many people who take themselves too seriously and don't understand the simple fact that the Universe can take care of itself. What each individual contributes into the world is returned to them three-fold. This is the law of Karma. What we do unto others boomerangs back to us.
Most of us know that if someone does us harm, they will suffer the consequences in this life or in the next. The Universe does not require our help to correct what we think are its "mistakes". The Universe does exactly what was pre-planned for each individual to erase their karmic debts and take them through lessons they are here to experience.
Revenge spells are used on the occasions that people feel they have to personally give someone a lesson. Little do they know that by interfering with the laws of nature, we are adding to our own karmic debt.
Our goal is to "live and let live". We must always remember that most of the people who come in contact are souls who have agreed to help us experience some pre-planned event. For example, if we go out to the grocery store and we see a poor man begging for coins. We ignore him and continue to walk, telling ourselves that he is a lazy bum. After a while, guilt gnaws on our conscience and we reconsider our behavior thinking, "I should've given him something; what if I became homeless?".
That is the realization that our souls are merely enacting a drama that we came into this world for.
