Mycroft is being completely counterintuitive. If they’re going to have any chance of beating Eurus, it’s John who should die. John knows it, Mycroft knows it, Sherlock knows it, and yet it’s not John at the end of the barrel. It’s Mycroft. For someone who claims to be missing a heart, the love that Mycroft feels for his little brother is stronger than the logic he reveres. And so despite all that brain power and potential Mycroft knows he offers if he lives, he chooses to puts it all aside and be a solider. To do his duty. To protect Sherlock. To be big brother.