Updated: I'm going to put the answer in spoiler tags, but if you're watching the first series, really don't look
The last episode of the third series just premièred this week, and it's only just resolved your question, so I'd say just watch it and see, don't spoil it.
It's a 3 series story arc and fundamental to the whole plot, so inadvertently revealing it will spoil the rest for you.
So I'll say as little as I can
There is a mutual attraction between Alice and John which comes from Alice being one of the few people to understand his actions at various parts of the stories where he goes over the line. Initially she has escaped justice as although John knows she is guilty of her initial crime, the police are unable to prove it. She is arrested by the end of series one, but for what is part of the finale.