Snakes hold the unfortunate privilege to be essentially the most vilified animals in most of history. Specifically in the West, snakes are associated with a whole library of terrible values, from treachery to murder. Due to the role on the serpent inside Garden of Eden, Christians associate the snake with temptation and the devil. Even so the Christian symbol in the snake is not the only one. Snakes are generally unassuming creatures who often leave humans alone. Relatively few snakes are poisonous, and few of the poisonous snakes are lethal to humans. But some exceptions exist, specially in Africa, Australia and the American West, for you are poisonous snakes nearly everywhere you'll find humans. Subsequently, snake symbolism is inextricably linked with poison.
Snake Tattoo
A sample snake tattoo with a skeleton addition
A scary snake tattoo
Older meythological systems often associate the snake with wisdom, fertility and knowledge. Snakes are associated with fertility mostly because of the phallic nature, with wisdom due to their careful movements, and knowledge through their association with assorted version on the Egyptian god Thoth, whose portfolio included writing, math and technology. Egyptian views from the snake were echoed during the entire Mediterranean cultures that predated Judeo-Christianity. Source tattoos.net.