Pennywise the Dancing Clown

Pennywise The Clown is the main antagonist of the film IT. It was a demonic extra-dimensional entity who would almost always disguise itself as a clown to attract children, so that it could capture and kill them as they were an easier target.

He is portrayed by Tim Curry.

IT
It was part of the disasters that occurred every 27 years in the Maine town of Derry.

IT was never against killing children. In fact, it actually preferred to kill children as they were an easier target. That was probably why it chose the form of a clown, a figure that both entertained and terrified many young children. It had razor sharp teeth that it could use to kill people.

As the film progressed, a group of children known as the Lucky Seven (also mockingly referred to as the "Losers' Club") banded together to kill Pennywise and end its murderous reign (after he had killed the brother of one of them) and apparently succeed after they tracked him down to its lair. However, Pennywise, being a lesser-aspect of a higher being, was not going to stay dead forever and it swore revenge on the gang for it's defeat before it vanished into nothingness.

27 years later, Pennywise kept his promise and came after members of the gang, who were all adults in the present day, to kill them. That prompted the gang to reform and battle Pennywise again in order to kill it yet again and save themselves from its wrath. At the end of the film, the gang did manage to defeat Pennywise again, but in that final battle, it took the form of a spider-like monster that was revealed to be his true form rather than the clown disguise (which it used for most of the film). It was killed when they pulled out his heart. After they killed it, the gang left its corpse to rot.