What was IT and what could IT do?

IT is a big disembodied brain. IT can control the minds of all the people on the planet Camazotz. Everyone on the planet is forced into total, synchronized conformity to the will of IT.
IT has captured Meg and Charles Wallace's father and is keeping him imprisoned. When Meg, Calvin, and Charles Wallace appear on the planet, IT tries to gain control of their minds.
We know IT is evil for several reasons. First, the planet is covered in darkness, and Mrs. Whatsit, Mrs. Which, and Mrs. Who are unable to visit it. Second, the children witness a young child making mistakes bouncing a ball. They later see the same young child in Central Central being "retrained" through cruel electric shock therapy to bounce the ball in conformity with everyone else. Third, IT uses lies and deception to try to persuade the children it has their best interests at heart in obliterating their free will.