In the comments on Mel's thread about the Wiccan lady's ritual gone awry, someone mentioned pentagrams as a symbol of evil. Here are some thoughts on that. I don't remember where I heard or read this, and I'm not asserting some great truth in it, but it makes a certain amount of sense.
The hexagram (6-pointed star) is a symbol for big-G God, because it looks a bit like the sun and because it can be seen as arrows pointing in all directions all the time (all-seeing, all powerful, always).
The pentagram (5-pointed star) is a symbol of mankind, because it is sort of shaped like a person.
Right side up, that is with one point up, the head rules the body. Mind over matter. Follow your brain, not your base desires. Good stuff.
Inverted, the body rules over the mind. Do what you feel like, regardless of the harm to others. Bad stuff.