Harry Potter Theory: IT's Pennywise Is A BOGGART!
3. A-Mortality
According to the mythology written by JK Rowling on boggarts, they cannot, strictly speaking die. They can be defeated or vanquished or made to disappear - however you want to say it - but that's no more than their powers being defeated in that moment. They cannot be killed because they were never really alive in the first place.
They are not ghosts or ghouls or memories of something living, their very existence is already formed when they are conceived. And the same can be said of IT.
We might see Pennywise "die" at the end of the first chapter, but he simply bounces back 27 years later. Who is to say how many times that has happened over the millions of years of its existence? It's not like defeating it turns out to be anything particularly spectacular. In both cases (IT and IT Chapter Two), it's just a case of being strong.
While IT might appear to be gone at the end of the second CHAPTER - its heart torn from Pennywise and crushed - the heart, Pennywise's body and everything physical about IT in that sequence is constructed in the imaginations of the Losers. They might stop his power and his hold over them, but they don't kill IT. IT cannot be killed. He bounces back because those are the terms of his existence. He too was never truly alive, so why should he be able to die?
And there are references to books after IT was published to IT being alive. Who could forget the chilling Pennywise Lives! Graffiti, for instance?
So let's talk about how IT is defeated once again and why that fits with what we know about boggarts...