The last few times I've watched HLV I've had this nagging urge to write out my thoughts about Sherlock and Mycroft in this episode, to get them straight before s4. Mainly I believe that Mycroft wanted Sherlock to remove Magnussen from his position of power, and manipulated him into doing it, but didn't expect Sherlock to kill him. Is this generally how people understand this episode? Or do people believe that Sherlock was acting completely on his own, that Mycroft really didn't want him to pursue Magnussen?

Someone has probably already explained it all, or maybe it's just blindingly obvious to everyone, but I can't remember seeing it detailed here. Feel free to jump in and tell me if I've missed something that contradicts my belief, or if I've just got it completely wrong

So, Mycroft wants Magnussen removed because he knows things that he shouldn't, but Mycroft can't be seen to sanction the murder of a private individual, a prominent media magnate who has personal ties with the PM, for no apparent reason — after all, who would admit that they are being blackmailed? Especially not Mycroft. Whatever it is that Magnussen knows, makes him untouchable. But Lady Smallwood is the first person to stand up to him (sort of). She seeks out Sherlock as the only person capable of stopping Magnussen.

When Mycroft discovers that Sherlock has taken the case, he tells him to stay away. Sherlock says that Mycroft is under Magnussen's thumb, and I think he is right — Magnussen has something on Mycroft, which is why Mycroft lets him go about his blackmailing in peace. And yet Mycroft knows what happens when he tells Sherlock to keep out of something — he told Sherlock to keep away from Irene, and look how that turned out. He's not likely to make the same mistake twice, so why wouldn't he use this as reverse psychology? He wants Sherlock to prove that Magnussen is blackmailing Lady Smallwood so that Magnussen's reputation is tarnished and he is sent to prison.

Magnussen has something on Mycroft (we may find out what is s4) and knows his pressure point is Sherlock. Mycroft knows that Magnussen knows this, and that Magnussen would use any opportunity to get at Mycroft through Sherlock.

How did Magnussen know about Redbeard? Mycroft has fed Sherlock's personal history to his enemies before — he could have let it slip to Magnussen deliberately, knowing that Magnussen would try to use it as leverage, to unnerve Sherlock, and then Sherlock would also have a personal reason to hate Magnussen (I'm still not sure who or what Redbeard represents, but it obviously strikes a nerve in Sherlock, and is listed as a pressure point in Magnussen's file on him). This would give Sherlock extra motivation to bring Magnussen down. Perhaps Mycroft knows how Sherlock feels about bullies like Magnussen? (Something from their childhood?)

Obviously Mycroft didn't anticipate Mary shooting Sherlock and complicating matters. It also complicates my theory, because I'm not clever enough to untangle that bit of the plot (how much Mycroft knows about Mary). But now Sherlock has even more of a reason to go after Magnussen.

Mycroft surely had Sherlock under surveillance when he slipped out of hospital to meet Magnussen in the restaurant, and heard that Sherlock was going to offer an information exchange "for Christmas." Getting Magnussen out of the picture becomes even more imperative, I'm just not sure if he wanted Sherlock to kill him.

The whole conversation between Mycroft and Sherlock at Christmas seems to be a setup. It was probably Mycroft who offered the illicit cigarette to Sherlock, and suggested they go outside to secretly smoke, away from the rest of the household. His excuse for himself is that he can't stand Christmas (just think of the way he is acting in the kitchen — and I mean acting). Mycroft brings up the subject of Magnussen, wants to know why Sherlock hates him. Sherlock replies that Magnussen attacks people who are different* and preys on their secrets. He seems to expect that Mycroft would hate him for the same reasons. (*This reminds me of the bobble hat deduction scene in TEH, when Sherlock implies that Mycroft is different, and therefore isolated/lonely)

Mycroft rejects this and tells Sherlock that Magnussen never causes too much damage to anyone important, and yet we have seen the newspaper article about Lord Smallwood's suicide. He mentions that Magnussen is occasionally useful to the government, a necessary evil, "not a dragon for you to slay." He says that Sherlock sees himself as a dragon slayer.

Mycroft then tells Sherlock that he wants him to decline the offer of the job in Eastern Europe with MI6, because it would "prove fatal in [...] six months" and on balance, Sherlock would "have more utility closer to home."

Sherlock asks, "How do I have utility?" and Mycroft replies "Here be dragons." In other words, he has another use for Sherlock right here, right now. At this point, Sherlock makes a very interesting series of facial expressions, like he is realising something and thinking about the implications. I think this is when he deduces that Mycroft wants him to go after Magnussen, despite saying the opposite.

While Mycroft is saying the cigarette isn't agreeing with him and that he's going inside, Sherlock is staring up at the sky. He's running it all through his mind now. He tells Mycroft that he needs "low tar" — a callback to the morgue scene in ASIB? All lives end. This is when Mycroft adds, "Also, your loss would break my heart." All hearts are broken. Is this a coded message for Sherlock to decipher? Is he telling Sherlock to go ahead with his plan to bring Magnussen down?

Such sentimentality is out of character for Mycroft, which is why Sherlock chokes on his smoke. What the hell is Sherlock supposed to say to that? Perhaps Sherlock is still not sure he is understanding Mycroft. Mycroft says "Merry Christmas" even though he hates Christmas. Another cypher. When Sherlock says "it's Christmas", it usually means an exciting case to solve, or someone is dead. Which meaning does Sherlock interpret?

Mycroft pretends that he doesn't know why he is being sentimental, and suggests there must be something in the punch. This is surely not coincidental; he must know what is going on. Mycroft is not shown drinking anything in the preceding scenes, but he does comment on Billy's presence at the family dinner. Being the smarter older brother, I think Mycroft is clever enough to work out that Sherlock has brought him to their parents' house for a reason. He is probably faking his drugged sleep when Sherlock returns to the kitchen, because he knows Sherlock's plan (why else does he leave his laptop conveniently lying around in the kitchen?). He doesn't seem drugged when a little later on he arrives at Appledore in the helicopter.

Magnussen was probably right about the GPS locator. If he was caught with the laptop containing top secret government files, they would have searched his vaults and arrested Magnussen. Like Magnussen says, Mycroft has been looking for this opportunity for a long time. This was probably Mycroft's plan and Sherlock has done the legwork for him. But Mycroft and Sherlock are both about to be brought up short with the whole mind palace thing. Neither of them deduced that.

I don't think Mycroft expected Sherlock to shoot Magnussen. I think Sherlock decided that at the last minute, when he realised it was the only way to protect John.

Now Mycroft is faced with two options: Sherlock is incarcerated for life for murdering Magnussen, or he is sent on a mission that will probably end in six months with his death, but which gives Mycroft some time to get his little brother out of it.

So is my impression that Mycroft had a plan to use Sherlock to get rid of Magnussen — a plan that went horribly wrong — a valid impression, or am I delusional?