If you’re feeling kind, lock her up; otherwise let her go. I doubt she’ll survive long without her protection.
I get the impression that Mycroft is himself a bit shocked by Sherlock’s cruelty in this scene.
Same impression, since the first time I watched the episode! Shocked and maybe also a bit worried. Like he were thinking: “This is the kind of thing I coud have said, but Sherlock?”. Sometimes I’m under the impression that, albeit maybe a part of Mycroft would like that Sherlock get persuaded to work with him on a regular basis, maybe even entering the Service, another part of him fears the eventuality that Sherlock might one day become similar to his elder brother.
And in this scene, I can almost hear this second voice speaking in Mycroft’s head: “Oh God, what if the resut of all this awful business were that my little brother becomes even more buttoned up, more cynical, more hard, and cold - more like me? What if the boy who was never able tu fully understand how a human being can kill another human being and not be in any way affected by it, had now become a merciless man, capable of coldly sending a now helpless woman to sure death? What have I done, in bringing him into this damned case?”
(Ok, ok, I’m probably being a bit over-dramatic, here… But yet, I hope I made myself clear, at least…)
That is, I’m under the impression that Mycroft is fully aware of the many differences between his personality and his little brother’s one, and even if he’d like that Sherlock became less reckless, and stopped putting himself into unnecessary dangers, and maybe became also a little more mindful of social conventions and social niceties, he wouldn’t like if Sherlock became a replica of himself. As I’ve said before, Sherlock, differently from Mycroft, is an extrovert person, is a much more lively person: he is curious (and therefore open, in a certain sense) about people; he can be rude and impatient while dealing with other people, yet he has chosen a profession which implies a lot of human interactions (the “legwork” that Mycroft accurately avoids…) and which is, all in all, a study in humanity; he throws himself at people and therefore, in ultimate analysis, at life, even if he does it with the tact of a rhinoceros… And I think that Mycroft, at the end of the day, wouldn’t like the idea that his brother lose this attude to get involved in things, in people - in life.
Mycroft himself is quite different - and I doubt he doesn’t realize it. He knows he is an introvert person. He knows he has retreated from life in many ways - partly in order to be better at his exacting work, partly (probably) out of his past experiences and education, besides his personal attitudes. He probably endorsed the nickname of “Ice Man” that Moriarty gave him - he feels to be, and wants to be, an Ice Man. But he is too clever to ignore that all this has a price - that he did also lose a lot, in his struggle to become impassive, cold, detached, always perfectly efficient, always in control, always able to protect others, always strong. He put on an armour, and this armour keeps him upright, no matter what, but at the same time it blocks the blood in his circulatory system, it makes him cold and insensitive and… (at least partly) dead inside.
(After all, the scene at the Manor, at Christmas Eve, when Sherlock calls him and we see Mycroft, soon before the ring of the phone, seated alone in a darkened room, staring at the flames in the fireplace, it’s almost an exact quote of the initial scene of Dicken’s The Haunted Man and the Ghost’s Bargain, where Professor Redlaw gives up all his sentiments and emotions in exchange for being rid of his sorrows, of his painful memories, of those weaknesses that impair, according to him, his perfectly scientific intellect.)
And I think that, deep down his heart, Mycroft doesn’t want that the same thing could happen to Sherlock, to that brother that, being seven years his junior, he will always see to some extent as a child - to that child who was always more lively, more enthusiast, more daring, more ALIVE than him.
All this, I think, is behind his shocked, worried expression in this scene, after having heard Sherlock coldly sentencing to death a woman he had developed a strong attraction to, and whose supposed death he had mourned, in his way, not so much time before. Mycroft is worried that Sherlock could one day become like him.
Oh, dear me! I’ve rambled again, and without even noticing!
To lighten the tone, I’ll quote here the excellent theory about Mycroft’s parting words to Irene that enigmaticpenguinofdeath posted on her blog:
“Don’t let the giant antique doors of my country house collide with any part of your anatomy as you show yourself out, Ms Adler.”