Shortly before Lily Evans was murdered by Lord Voldemort, Snape changed sides and became a member of the Order of the Phoenix and double agent during the Second Wizarding War.
In addition Harry later named his second son Albus Severus Potter in honour of him. Severus, whose father was neglectful and possibly even violent, began to identify with his mother's family and created a secret nickname from his mother's maiden name, calling himself "the Half-Blood Prince." His unhappy relationship with his father may have been the origin of his disdain for Muggles.
It is implied that Severus was friendless and uncared for by his parents.
This area of town was near a dirty river and full of dilapidated houses, disused factories and broken down street lamps.
Through the rest of his life, Severus continued to return there when he was not at school.
Snape developed a passion for the Dark Arts at a young age, which increased as his desire for revenge grew stronger.