Study of the Dark Mark

A bright green skull with a serpent for a tongue. You Know Who’s Mark has been famous for centuries. The green serpent symbolizes Slytherin and dark wizards. Yet there is more behind the symbolism that lets us analyze He Who Shall Not Be Named’s personality. Symbols show many things about people, and why they chose these marks as their symbols.
The snake is a symbol of healing powers which can explain Voldemort’s obsession with immortality. The snake is one of the twelve zodiac signs. People born the year of the snake are cunning and don’t stop to achieve what they want, even at the expense of others. When they fail once, they will go through great lengths to not make the same mistake again. This explains Voldemort’s thoughts with Harry and the prophecy. They are also very distant and do not have any close friends. They are power hungry and crave fame. A snake will face danger head on and relentlessly. This describes Voldemort completely which shows why he chose the snake to be his powerful mark to face the world.
The skull is the universal symbol for death. They symbolize death, pain, torture, and mortality. Voldemort obviously chose these two symbols to show himself (and his Death Eaters) conquering his enemies. The skull would be his enemies, mortal, dead at his hands, probably tortured while the snake is the dark side. It is alive, immortal and proud. This is the dreaded symbol of Voldemort and his Death Eaters. They send it into the sky over the building every time they’ve killed someone. They do this to show their power and immortality.

Sources: http://en.wikipedia.org/wiki/Skull_(symbolism), http://www.chinavoc.com/zodiac/snake/person.asp