Why do witches cause murders?

The question makes assumptions.

There is no evidence to suggest those that follow a religion that includes witches as a title, such as Wicca, cause murders.

In addition; The medieval concept of witches is simply ignorance and fear, especially of things they were unable to understand.

Such concepts are propagated today by juvenile novels, TV programs and movies. Much of which is incorrect and a corruption of the true facts.

For instance there was never any huge 'witch hunts', nor were witches burned at the stake (though some few were), most were simply hung.