Similar to Freddy Krueger, Pennywise uses fear to terrorize and murder his victims. It can be whatever fear that person has. It's also noted that both characters use the personal insecurities and grief of their victims to make them more vulnerable and easier to kill. See more

Goofs

When the group go back to Bens room, Beverly closes the door and you see a New Kids on the Block poster on the back of the door. She and Ben exchange smiles, and then she reopens the door to hide the poster from the others. In the next shot, the door is closed and the poster is no longer on the back on it. See more