Severus Snape was the head of the Slytherin house because he was a former Slytherin. He was born on January 9, 1960. He died on May 2nd, 1998, at the age of 38. His boggart was Lord Voldemort, and his Patronus is a Doe. His parents were Tobias Snape and Eileen Prince. During his time at Hogwarts, he was known as the Half-Blood Prince because he was a half-blood and his mother's maiden name was Prince. He was also bullied by James Potter during his time at Hogwarts, and hw\e was in love with Lily. He joined the death eaters to impress Lily, and he then quit and joined the Order of the Pheonix just before her death.
SPOILER ALERT
The reason he killed Dumbledore is that Dumbledore told him to because he knew Draco wouldn't do it and would be killed by Voldemort.

-Luna Erudite