"The Juews are the men that will not be blamed for nothing..." Why would a group be targeted for blame?

Most likely in an attempt to throw suspicion in another direction if it was written by the killer/s. Or it was written by an uneducated anti-semite to try to bring down the authorities on people that the writer didn't like. In which case is would more likely have been written by someone other than the killer/s. It's not really known exactly who wrote that message. So the motive behind it is really up in the air.