Further to the point I was trying to make yesterday, isn't the fact that just 2 of the Canonicals were known to have been engaging in the type of business that facilitated the goal of the murderer/mutilator something that should be strongly considered?
We really don't know why Liz was in the passage, nor why Kate turned the opposite direction of John when leaving the jail, and we can see that Marys circumstances were much different than any previous Canonicals. We really have only 2 women who were known to have been soliciting...something which I believe is a key to these cases. Prostitutes, or Unfortunates prostituting for their bed, gave the random opportunistic killer 2 things....anonymity and privacy. I believe Pollys murder represents the first time he tries to free these demons that have been haunting him. He cant wait any more...and as a result he strikes before he has the secured privacy. He learns from this and is anxious to try again with more self control.
Summary...the man who killed the women while they solicited used their own economic vulnerability to get them alone. He didn't have to "act" really, he could just allow himself to be taken by the arm. Even a socially awkward man could have pulled that off.