No, it is a film based on a fictional novel by Stephen King. There's a real standpipe which King used as his inspiration for the novel, but he gets a lot of his ideas from visuals that he sees/recalls. But none of the story is true.

The backstory of Pennywise is tied to an ancient cosmic entity referred to as the "It" or the "Eater of Worlds." This entity is from another dimension and has existed for an extremely long time. It comes to Earth approximately every 27 years to prey on the children of Derry. He/It didn't turn evil  it simply is evil.