One brain to rule them all, one brain to find them, one brain to bring them all, and in the darkness bind them! IT is the evil genius of Camazotz bent on world domination, and he's all brain:
A disembodied brain. An oversized brain, just enough larger than normal to be completely revolting and terrifying. A living brain. A brain that pulsed and quivered, that seized and commanded. No wonder the brain was called IT. IT was the most horrible, the most repellent thing she had ever seen, far more nauseating than anything she had ever imagined with her conscious mind, or that had ever tormented her in her most terrible nightmares. (9.124)
If you're going to get into the evil mastermind business, you might as well go all the way. But why does the book make its most evil entity into a giant brain, and nothing but? Well, there's the obvious gross-out factor. But IT also seems to be evoking the brain's status as the seat of reason, as opposed to the heart's monopoly on feeling. Looking at IT this way, Meg shouldn't have needed cryptic statements from the Mrs. Ws to figure out IT's weak spot: heartless, all IT needs is love. IT, and the society IT creates on Camazotz, could be seen as what happens when reason runs amok, untempered by the heart's compassion. Sure, it may make a certain amount of sense from a purely rational point of few to kill off sick people, but reason shouldn't be the only factor at play in making decisions. IT shows that supreme power can also be supremely limited when it privileges one approach at the expense of all others.