In the end, it's a remarkably easy decision.
Mycroft Holmes in an exceedingly intelligent man. He is intelligent in a way which both undermines and compliments his brother's intelligence. Mycroft had not been surprised to learn that James Moriarty had labelled him 'The Iceman'; it is his job, after all, to be as coldly efficient as is possible. Mycroft's mind is a highly organised place. He never makes a decision before thinking it through carefully in order to reap the maximum benefit to aid his steady climb up the social ladder. Mycroft is the brain behind the masterplans, the engineer behind the elaborate schemes. In many ways, he is the polar opposite to his younger brother. Where Sherlock is chaotic, Mycroft is systematic. Where Sherlock plunges into situations headfirst and careless of the consequences, Mycroft follows at a slower pace, always judging the possible outcomes as they present themselves. If Sherlock is the storm, then Mycroft is the calm before it. It has always been that way, ever since Sherlock was old enough to understand that his actions had serious consequences and chose to ignore them anyway. Sherlock runs where Mycroft walks. Sherlock leaps where Mycroft steps. The difference in temperament is an irrefutable fact. Sherlock will never be calm and sedate. Mycroft will never be careless and wild.
But then the gunman, cornered against the back wall of an abandoned warehouse, raises his gun towards Sherlock, and suddenly none of that matters in the slightest.
Mycroft does not think. Mycroft does not consider the consequences. All he knows is that there is one very possible outcome of this situation, and that outcome is Sherlock bleeding on the ground. That outcome must be stopped whatever the cost. It does not matter that this is not the correct policy. It does not matter that he himself could end up hurt. In that single moment, as the barrel of the gun levels towards his younger brother, Mycroft's mind is wiped blank of intricate plans and careful considerations. The scheming and plotting is replaced by an instinct hard to overcome.
SHERLOCK. DANGER. PROTECT.
Lestrade is yelling, and he can hear John Watson's voice ringing out behind him as he surges forward, but Mycroft ignores them. He slams into Sherlock, the impact throwing his brother off his feet and sending them both tumbling to the ground as a gunshot echoes out. The gun clatters to the ground and the criminal tries to run. The police are on him before he can. Mycroft is aware of the events unfolding behind him, though he pays them little heed. Breathing hard, he pushes his hands against the floor to give himself some leverage, and sits up.
Sherlock looks... dazed. He's staring at Mycroft with something akin to wonder in his eyes. Mycroft is suddenly reminded of those same eyes gazing out of a five-year-old's face, back when that five-year-old believed that Mycroft hung the stars. Then Sherlock blinks, and the moment is gone. There's a graze on his cheekbone from where he hit the ground; Mycroft reaches out to touch it, but Sherlock's gaze slides from his face to his shoulder, and his expression becomes one of concern.
'Mycroft, your shoulder - '
'Hmm? Oh.' Mycroft twists his neck to stare at the bloodstain on his shoulder. 'Oh.'
Sherlock clambers to his feet, dislodging Mycroft in the process. Mycroft sits on the ground, uncaring of the indignity of it for once. He hadn't noticed before, distracted as he was by Sherlock, but now he comes to think of it but his shoulder hurts rather a lot. The bloodstain is small; not too much damage, he notes, only a minor flesh wound. Lucky. Another inch or two and the bullet would have hit his neck. Mycroft chooses not to consider the fact that he has automatically accepted that as a preferable outcome to the one where the bullet hit Sherlock. His brother is talking to someone nearby; a glance confirms that it is John Watson. John's entire face seems to have drooped in relief, the tense, anxious lines in his forehead smoothing out in the presence of his best friend. He has a hand on Sherlock's arm as though to make sure he is actually alive and well before him. But Sherlock is pulling him over to Mycroft, gesticulating and talking ridiculously fast. Mycroft waits patiently. John squats down beside him; Sherlock doesn't, but he stands close by. Mycroft can sense his brother's eyes on him. As John begins to examine him, telling him he was lucky, and it was barely a scratch, and what a narrow miss it was, the sensation of Sherlock's eyes on him, unchanged from eyes of the baby brother he remembers from childhood, gives him comfort far more than John's reassurances that he'll be fine.
He is fine, of course. John is a skilled doctor, and after Lestrade directs him to some medical supplies, he cleans and swabs the wound with an cool efficiently Mycroft recognises and admires. Mycroft remains calm throughout the process, though even he cannot hide a wince as the antiseptic stings his flesh. Sherlock mocks him, without heat, and John reprimands him absent-mindedly. Mycroft feigns a superior look and Sherlock pretends to smirk. It's easier to slip back into their normal pattern of bickering than it is to face what they both know is coming.
John leaves to replace the medical kit. Sherlock regards him from where he is standing, a metre or so away, hands deep in the pockets of his greatcoat. Mycroft recognises it as a sign that he feels awkward, but does not mention it. Sherlock has taught himself how to hide emotions for so long. There is no need to make this any more difficult for him.
'What you did,' he begins, glancing at the ground and then dragging his gaze back up to Mycroft's face unwillingly, as though he'd much prefer to study the ground as he spoke. 'It was – er. It was.. good. Brave. You didn't need to - what I mean is...' He trails off, eyes slipping downwards again. It is with a visible effort than he makes himself continue. 'Thank you.'
Mycroft stands up, holding his injured shoulder steady with his left hand. He steps towards Sherlock and sees something close to panic flash in his little brother's eyes. Sherlock really has closed himself off from emotion, to an extent greater than even Mycroft realised. He is getting better, though, under John's influences; it is one of the many reasons Mycroft remains eternally grateful to the man. But this is uncharted territory for Sherlock. Mycroft represents childhood and adolescence; he is a living, breathing reminder of the life Sherlock has left behind. This, however, is not the time to pander to Sherlock's emotional insecurities. Some things need to be said. Mycroft carefully raises his right arm, conscious of jarring his shoulder, and sets his hand on Sherlock's shoulder. Sherlock watches him guardedly.
'You're welcome,' he says, simply. He recognises a hint of amusement in Sherlock's eyes and shares it with a slight smile. 'Thank you' and 'you're welcome' are words more suited to everyday, mundane tasks. They are not words used when somebody has just saved the life of another person. But it is all they need. Sherlock knows that Mycroft would do it again in a heartbeat, and he doesn't need a reason for it. Sherlock may have John now, and Mycroft may have his position and his wealth, but in a peculiar way, they are all the other has. Mycroft lowers his hand, and maintains eye contact for just a moment more, before turning away towards the car where Anthea waits for him.
'Protectiveness comes with the job title of 'elder brother', I'm afraid,' he comments, as Sherlock falls into step beside him.
'Over-protectiveness, you mean.' Sherlock's tone is teasing, but empty of the viciousness he often resorts to around his brother. Mycroft is glad of it.
'It is only necessary when the younger brother insists in placing himself in harm's way on a regular basis,' Mycroft responds dryly.
'Why were you even here?' Sherlock needles him. The familiar ground of goading Mycroft is clearly comforting to him. Mycroft finds he doesn't much mind. He is fighting to restrain a smile. He wins, of course; it would not do to let Sherlock suspect that he enjoys their quarrels. The thought is abhorrent.
'As you so frequently point out, I am 'the British Government'; I hardly need explain to you my reasoning for everything,' he says loftily. They have reached the car. Anthea is eyeing them with open surprise. She is used to witness arguments and fights, Mycroft knows. The ease and light-heartedness with which the two of them are bickering must startle her.
'You don't need to explain,' Sherlock scoffs, standing by as Mycroft climbs into the car. 'You were spying on me. As always.'
'Protectiveness, little brother,' Mycroft reminds him, and Sherlock's mouth twitches briefly in a half-smile. Mycroft returns it in kind.
His journey away from the warehouse is much more relaxed than his journey there, despite the fact that he now has a bullet wound in his shoulder – well taken care of; Mycroft makes a mental note to thank John in person as soon as possible. They have been travelling for twenty seven minutes exactly when Mycroft's mobile vibrates. He picks it up.
I meant it.
Mycroft's mind conjures the image of Sherlock's uncomfortable 'thank you'. He wonders, vaguely, when Sherlock last thanked him for something in complete sincerity, and not out of sarcasm, before replying.