Pagans and witches and wiccans (oh my) follow what they call the Wiccan (or Witches') Rede, or the Threefold Law. Basically anything youdo comes back to you threefold; whether good or bad. It is meant to make newbies think twice about working manipulative magic on another person. Obviously it is simply a version of the good old Golden Rule.
We should try to do good in the world as often as we can. Not for its reward, but because it is right to do so. Anything less makes us selfish, or sociopathic.