Reading this book as an adult is a very different experience to reading it as a preteen. The extra life experience makes this book so much deeper than the "supernatural killer clown novel" I viewed it as when I was 12.
The very fact that the humans in the novel, the citizens of Derry, are willing to partake in the evil buried beneath the town is the true horror of the novel. Not every citizen is like this; Mike makes note of the help and compassion received after the burning of the Black Spot, and Bill makes note of the rescue efforts after the destruction of Derry during the storm.
IT infects Derry and the people within, but it's not universal. IT is able to influence the people who are more disposed to cruelty; Al Marsh, Henry Bowers and Co, etc. And this is the real evil. IT is the epitome of evil, Evil itself made incarnate. IT has a reason and a motive to be evil; it is Evil itself. But humans don't have that reason. Humans aren't born evil. We are capable of it, but we are also capable of being good and of being neutral. When people choose to do evil, to inhabit evil, it is more monstrous and more terrifying than the monster; the monster doesn't have the option to be different, but we do. And that makes the humans scarier.
And this is not confined to the page. We know this to be true in real life. King captures this recognition of the true horror of evil and the human condition beautifully.
And the symbolism in the child at the end of the novel is gut punching; running through the destruction of the city chasing a paper boat down the flooded street. But the destruction, while looking terrible, is due to the death of the Evil that, in other circumstances, may have killed this boy in much the same way that Georgie was killed. The book begins and ends with a child playing innocently with a paper boat, but through the efforts and sacrifices of the Losers, this time, all will be well.