At the visualize farm we know from "Red Barn",  two red headed women kill a member of Visualize.  According to the killer's story, her mother was being attacked.  The daughter killed a man, in self defense.  Mysteriously another murder was occuring; in fact, two murders were supposedly happening.  
I suspect that there were no other murders happening.  McAllister was called in as a friend by the mother of the shooter.  She needed police assistance.  She didn't want her daughter to go to jail, so she gets McAllister to help her out.  McAllister might be her ex-boyfriend or the girl's father.

McAllister shows up and gets rid of any witnesses.  However, one witness escapes (mysteriously).  Only people familiar with the farm knew about the underground room.  The mother knew about the room, and we know she did.  In the story, the daughter reveals that her mother knew about the room - since she had been out there (before).  

In order to track down this alleged witness, McAllister left an ensign.  He draws a smiley face in blood to make a trademark.  He plans on getting some publicity, so he can intercept the witness.  If the story got to the press, the surviving witness could come forward.  When he or she came forward, McAllister would be able to intercept the witness.  He would use his police badge to get information from other officers.

Since the attempt was unsuccessful, he needed to murder again.  He planned on luring out that witness, because he didn't want to be blackmailed nor arrested.  Truthfully, Visualize could have been blackmailing McAllister, but it is unlikely.  More murders meant the surviving witness could come forward, and he or she could solve the case.  More murders were a way for Red John/McAlister to try to get info about the surviving witness to his murders.

The reason for the victims being red headed is simple.  The girl who did the original murder was red headed, and her mother was red headed (or are red headed).  McAllister wanted to stimulate the witnesses memory of the events from that night, so they would come forward with his or her testimony - which could put McAllister in prison.

If Mcallister did get backmailed, the blake association could be a group of police officers being blackmailed.  McAllister could be a "drop", in the bucket.  However, there aren't a lot of criminal law enforcement, even though the B A  is a large organization.  It seems unlikely that many police would be a part of it, but they could be bribed.  Timothy Carter had a "slave" locked up, in his basement.  If she was pimped out to politicians, judges, police, or FBI , those "johns" could be blackmailed.  Red John might be a name chosen for being one of many "johns".