The pathologist wanted to get his own back on the Scottish guy (after the first death/stabbling of the woman who looked under the bed). He therefore took the body of the other woman (the one with Oz on her arm) and left samples of her DNA aroung several other murders, making Scottish man think they were all connected by someone they named the Wraith. The swabs all went through the lab where the pathologist worked, so he just kept returning results saying that all the murders were linked.
I think.