It depends on what you define as "witch". Are you referring to someone who engages in new age and occult practices as part of their life, or someone with a magic wand and pointy hat?
I've read books on the subject of Paganism and modern day "Wicca", and one of the practices they uphold is similar to "do unto others"- they believe in karma, but when the idea of "magic" is involved, it is 3 times as powerful. If you attempt to be a "bad witch" (hexes and the sort), not only might it not work, it could potentially come back to haunt.
Wicca is really all about what kind of energy and attitude you send out toward others. If you treat others with kindness and respect, and send out positive thoughts into the universe through ritual and prayer, they believe it will reward you in the future.