The Heroism and Death of Eddie Kaspbrak
In every version of It, Eddie Kaspbrak meets a tragic and heroic end, but the specifics of how he gets there are different between the book and the movie. In King's novel, Bill and Richie are attempting to defeat It using the Ritual of Chüd, like they did as kids, but they're too weak and are in danger of losing. Eddie, who has known since childhood that his trusty asthma inhaler is a placebo, steps in, using the strength of his imagination to insist that the inhaler is filled with poison. He sprays "poison" down It's throat, weakening It and saving his friends, but in doing so, It bites off Eddie's arm, causing him to die of blood loss in Beverly's arms.
In the film, Eddie uses a fence spoke to defend Richie from It, after his friend is caught in It's "deadlights." Beverly previously told him that the spoke "kills monster," but only if he believes it does. Eddie repeats this to himself several times, making himself believe that it is indeed a monster killer, before javelin-ing it into It's throat, causing it to release Richie. Eddie rushes over to see if his friend is all right, but when he turns his back, the wounded It uses one of its long talons to skewer him through the chest. Eddie hangs on for a few more painful minutes, giving the Losers the idea to "make It small" so that they can kill It but then slips away before they can tell him they've won.