Pennywise, the main antagonist in It, is meant to be a shapeshifter who has taken the form of a murderous clown.The act of using a childs entertainer as a way of attracting children to then kill them can be considered one of the most vile acts any creature can do.
What is most terrifying about the idea of a killer clown is the fact that it happened. John Wayne Gacy, also known as Bobo the clown, kidnapped and killed little boys around his neighbourhood. More often than not he chose his victims by being hired by the family of the child and then kidnapping them at a later date, hence how he was caught.
Stephen King’s stories have always been a bit of a horror fest but the fact that this one in particular can be connected to a real life event makes it a real thing to be fearful of.