Now you see me, now you don't

Never could the boys have realised the implications of seeing the naked man rising up from the gravestone, on the next 150 years. Nor could the police have appreciated the importance of the puncture wounds on the victims throats, or the lack of forensic evidence. What were the puncture marks? Why is the killer concentrating on Bucknall and Hanley for most of his victims? What is the significance of the lack of forensics? How and why does the killer keep reappearing years later?