Eurus and John 

 (Sherlock meta by Ivy Blossom)

One of my lingering questions about S4 is about Eurus’ texting affair with John. Why did she do it? We aren’t given any answers, so we can only extrapolate.

What we know is that Eurus has been gearing up to meet Sherlock for quite some time. We also know that she has a history of getting rid of whoever is closest to Sherlock. It would make sense for John to become Eurus’ new target. Is he?

What did Eurus intend to happen when she gave John her number? Was she aiming for a texting-only relationship that could be conducted from Sherrinford as easily as from a bus stop in London? Or did John just fail to entirely take the bait?

Eurus is trying to seduce John, and from the sounds of it, sex with Eurus is violent and ends in death. If John had given in, would Eurus have killed him and given Sherlock a rhyme to help him find his remains? Or would she just have put him in that same well (as she finally does in The Final Problem?)

Or did Eurus intend for John’s text-only cheating to destroy him from the inside out, slowly erroding his sense of self-worth, until he’s convinced that he can’t be Sherlock’s trusted companion anymore? Is John’s pain in The Lying Detective the adult version of leaving a boy to die in a well, forcing Sherlock pull out all the stops to try and save him? That seems like a stretch, but Eurus has demonstrably set stranger events into action before, and that is in fact what happens.

It certainly lets John off the hook for his bad behaviour. Does John’s actions trouble him so badly because it actually is out of character and against his nature, but he couldn’t stop himself for reasons he doesn’t understand? The story gives us information from which we can surmise that this could easily have happened, but the story doesn’t confirm it. John’s relationship with Mary was in trouble, so his behaviour can be reasonably, if distastefully, explained in an alternate way. But the circle is never squared, and a finger is never directly pointed Eurus’ way for this, so it’s hard to say definitively.