I firmly believe that it was something within John's own psyche that caused him to kill prostitutes.

I...kind of don't. But I do believe that John and the Energy Being reached some kind of impasse (unbeknownst to John) wherein they just killed whoever was easier and where the greatest potential for violence lay. But at the same time, John did come up with the idea (self-preservation, access, ease, etc), so there's that.

I do love that he always asks her "Why?" and sounds so very betrayed by it when he find outs it's not to kill him.