Want to see some symbolism?
Eurus is The Final Problem. Literally. She is the embodiment of the entire episode. She is “burning the heart out of Sherlock”. We, the viewers who observe, are Sherlock. Mark Gatiss is Mycroft, obviously.
Mark had a trick up his sleeve. He’s kept it hidden in a remote place for a very long time. He kept it behind glass – behind elephant glass – and we had no idea it was there. Almost a secret twin. Almost.
This trick is The Final Problem. A trick so clever it had to be hidden. A trick that sees no emotion, but only registers the end result. This is why Eurus only cares about “The ends justifying the means”.
“You look unsure. You’re not used to feeling unsure.”
She says this to Sherlock, but that’s really meant for the audience. Everything we know about Sherlock Holmes and everything we expected to see is devoured by Eurus. The Final Problem made us unsure.
She attacks Sherlock because she wants to hurt him. The glass is gone and the secret is unleashed. The Final Problem has started.
Next, Sherlock and Mycroft are locked in a room. Eurus conducts “human experimentation” and Sherlock experiences it, “like a lab rat”. So. The Final Problem is locking us up with Mark Gatiss, who gave us the lid to the “I love you” John-sized coffin. We throw a tantrum because this whole thing is ridiculous and unnecessary. But we have to be soldiers. Mark spews venomous words and we turn the gun on him. “Every cause has martyrs. Every war has suicide missions, and make no mistake, this is war.”
Mycroft/Mark later gets reamed for not being good enough, for screwing everything all up.
The only way for the viewer to make peace with The Final Problem is to see it through to the ending, to give it the care it needs. This is a huge risk, all of the eggs are in this basket – The Final Problem, though psychotic and harmful, needs the audience to be patient and attentive, if it is to land the plane. We know it’s never the fall that kills you – it’s the landing.
The Final Problem knows how to play the game. It’s taught the audience how to play. It was a gift. Just like Sherlock’s violin.
When Eurus plays alone, she can’t tell if it’s beautiful, only if it’s right.
Because she’s meant to play a duet.