"Bitterness is a paralytic. Love is a much more vicious motivator."
Sherlock could honestly admit that he didn't realize quite how true those words were, when he spoke them years before.
But there he is on Magnussen's patio, John at his side, wind whipping through his hair, under his coat. "Sherlock, what do we do?" John demands, yelling over the sound of the helicopter.
Oh, John. Sherlock already knows what must be done, what he must do. He's not fond of it, mind you. Even though he has never shied from violence, when it arises, he would prefer not to seek it out. He'll do it anyway, of course, in a heartbeat.
Damn the consequences. Damn Magnussen, and Mycroft, and frankly, damn Mary too. The letter M is ruining my life, Sherlock thinks, almost giddily.
"Nothing! There's nothing to be done." Magnussen shouts, and it is so obviously clear that he knows he's won. That he never even considered Sherlock and John to be players in his game. "Oh, I'm not a villain , I have no 'evil plan'. I'm a businessman, acquiring assets. You happen to be one of them. Sorry, no chance for you to be a hero this time, Mr. Holmes!"
Sherlock allows himself one moment to breath. Whatever he might think of love - useless sentiment, illogical, dull - he knows that it's the reason he's going to do this. The ramifications for him, for his life, will be steep, there is no getting around that, but he's going to do it anyway.
It's John, of course. Steadfast, brave, wonderful John, who even now, when all seems lost, looking at Sherlock as if he is the smartest person in the world, and will come up with the solution to their problems at any moment. The doctor is right, of course, but John wouldn't come up with this as a viable solution, not with the entire British government watching. But Sherlock does. It's the only way to keep John safe, and he will do it.
The detective takes one last glance at his friend, a man he loves with all his heart, then turns back to the bespectacled politician. " Oh, do your research . I'm not a hero, I'm a high-functioning sociopath." He pauses, one more moment, just for him. "Merry Christmas!"
He fires, and knows that John is worth it.