So why did the murders happen?

No, they only murdered the ones that found out about them, i.e. the one in the beginning who fled to her friend's house and Suzy's friend whose name I can't remember off hand. They killed the blind piano player because the head mistress' nephew was afraid of his dog and complained that the dog had bitten him, which the man said was impossible, but they killed him anyway after firing him.

They used their sorcery to attain wealth and power and just stopped anyone who got in their way.