I think the most interesting thing about Suspiria is that the witches who run the ballet school are essentially breeding their victims. It isn't like any other story where a serial killer or killers choose their victims at random. According to the witch expert with whom Suzy Bannion speaks, witches must kill in order to live. Their reasons for killing aren't crimes of passion or based on revenge. This is why I believe it is so important that they keep their victims all in one place so they can have control of them from the time they join the ballet company until they are killed. That way, the witches can cover up these crimes without having their tracks traced.
Additionally, the witches get to know their victims which makes the entire concept all the more creepy. The women who run the ballet school are in contact with their dancers on a daily basis and yet are still willing to kill them. I think a part of this has to do with the fact that it is a high-strung ballet school where girls aren't just going to leave if something slightly strange happens. They've worked hard to be in this prestigious academy and in such a competitive field as ballet, leaving just isn't an option. The ballet school is undoubtedly a trap through which the witches can pick and choose their victims.