Often, young and old make the requests alike that have no working knowledge of the various traditions of Witchcraft, nor the lifestyle, faith, and symbolism involved. This is a good time, to take a deep breath, ground, center, and then start my own questions. The answer is very telling about the individual doing the asking. They're very personal, and an individual expression of intent and focus based on the faith and traditional guidelines set up by the individual casting them. Since they involve a spiritual intent, you have to have a working knowledge of the basics first.
Intent is the cornerstone of every spell that's cast.
This guideline, of course, will vary with the tradition being practiced, as not all Witches are Wiccan, but it is a good idea to keep in mind that if you intend to harm someone, then your asking for that harm to come back to you at least three times over. Experience has proven over and over again that Karma does indeed keep tally. You are responsible for the behavior you exhibit, for your reaction to any scenario, and for the road you take to deal with it. Spell work is not a short cut to the trials and tribulations that you encounter in your daily life. If anything, when you cast a spell, your setting into motion even more focus on the situation, calling energy into play that you had better know all about before summoning, and be very aware of which path the returned energy is going to take.