The Law of Three
When willingly offered, gifts are said to come back threefold. If you deliberately harm or manipulate another person through the use of Magic (black or otherwise), or through any form or thought, deed or word (whether performed with malice, evil intent or not) you will pay for your actions by having the bad deeds return to you threefold.
By the same token, when you do something good or positive (either through Magic or other actions in the mundane world) the results will be threefold, good Karma returned to you.
"What goes around, comes around" has been proved true many a time. This concept can be seen in many religions and belief structures. The word "just is defined as "that which is merited or deserved". With that definition in mind, you can see why wiccans believe in the exercise of "justice" over "revenge". We only need to ask the Goddess & God for "justice to be done". Since this is inline with natural laws we can rest in the knowledge that we are asking in the correct manner for the situation to be taken care of. Then we can release it to them and go about our business. "Revenge" on the other hand, is defined as "an urge to get even" (we have all been there at some stage in our lives) (DO NOT GO THERE!) It is difficult sometimes, especially when our loved ones have been hurt, never the less you are a wiccan, trust that the Goddess & God will take care of it, trust yourself, trust in the threefold law, be strong in your beliefs. How we decide to respond to circumstances tells us alot about ourselves, what we say and what we do. It can be a valuable lesson for you.