So, I've been doing some more projecting thinking.
The more I go over it in my head, the more convinced I am that the whore James and Helen found was John's work. I think I'm 99% sure. The timing of it was just too perfect.
John had only had a relatively short time to learn to control the creature at that point, and you can bet it's not happy about it. You could see how much effort it took him to suppress it for even the few seconds he needed to prove his innocence to Past!Helen. That thing was pissed. Plus, I don't know if I'm crediting the creature with too much awareness, but I would assume it's not a big fan of Helen's. Killing would have been the only way for John to appease it. The murder would have happened almost right after he teleported away, and it just makes so much sense for that to be the first destination to pop into his head.
The main thing, though, is that gash on her cheek. There's nothing in the Spring-Heeled Jack legend about him cutting up people's faces, and why else would they have her cheek cut in just that spot? The same spot as John's scar from Helen's bullet? It's like the creature forced him to take his feelings for Helen out on another woman. Especially this is presumably the first life John has taken since Helen shot him. (Though the show has already given John more kills than were ever definitively attributed to the Ripper, so it's hard to be sure in that regard.)
So, here's where I'm at. The original murders were John. The copycat murders were Spring-Heeled Jack. But that last one? John again. I'm 99% sure.
My next task is to figure out whether or not Helen figured it out. I want to believe she did. Because that would mean that she deliberately and calculatingly kept it from James to protect John. Probably to protect both of them. And that would just make everything infinitely more amazing.
I don't know why I'm so desperate for all this to be true. But God, I am. I really, really am.
There's probably more to come. I seem to have misplaced my brain's "Off" switch. Or John broke it. Whatever.