In ancient times the regular pentagram was a symbol for Venus, both the goddess and the planet.
When most people see a pentagram they think of Satanism and witchcraft. There is no easier way for a heavy metal band to show just how bad they are or for a movie to hint to sinister characters. The association with evil is quite recent. In fact, in the Middle Ages the pentagram was meant to scare away demons, not summon them. It was also a Christian symbol denoting the 5 wounds received by Christ.
The famous Greek philosopher Pythagoras saw the was drawn to the geometrical shape. The fact that you can draw it in a single line and start exactly where you finish made it the perfect symbol for life and health. Ancient Druids and Pagans use the geometrical sign to symbolize the elements.
Our modern interpretation of the pentagram as a symbol of evil can be traced to the occult author and ceremonial magician Eliphas Levi. He popularized the pentagram which was standing on one tip. Quite often the pentagram was drawn around a goat, with the bottom tip being its chin, the two side tips the ears and the top tips the horns. Later occult depictions took the sign and ran from there. After its use on the cover of “The Satanic Bible” the pentagram was etched as an evil sign in our collective mind.