I don’t know about you but, erotic fantasies aside, I was quite stunned when Sherlock attacks his brother and pushes him against the wall.
On one hand it seems to throw out of the window how Sherlock was starting to see his brother more as a human being and less as a useful annoyance. On the other hand, it seems so very out of character. We never see Sherlock resort to violence, unless he is attacked first. Whenever he has a conflict with someone, including Mycroft, it is always an argument. Even when he is drunk, it isn’t until someone throws a punch at him that he tries to fight back. Of course, different substances have different effects. But, since the moment John finds him in the crackhouse, Sherlock shows no signs of aggression, repressed or otherwise. Not when John is berating him, not when Molly slaps him, repeatedly. Sherlock only seems to be annoyed with them. He doesn’t get angry until he realises that his brother is at Baker Street.
So what did Mycroft do to deserve such an extreme response? Was it his refusal to deal with Magnussen? But their argument over him takes place within their familiar dynamics. Even when Mycroft, falsely, tells Sherlock he will personally intervene if he doesn’t drop the case, Sherlock remains calm and dismissive. Furthermore, before mentioning Magnussen, Sherlock allows his brother to believe he has drugs in his bedroom in order to avoid him seeing Janine and realising how far along his plans for Magnussen are. He wouldn’t have done that unless he expected his brother to oppose him in this. So, he isn’t happy with Mycroft’s passivity in regards to Magnussen, but it is definitely not unexpected. Therefore, I don’t see it working as a trigger.
And what else is there? Besides the argument about Magnussen, all we see is Mycroft worrying about Sherlock being back on drugs. And everybody is and has been doing that for at least an hour, why would it be any different when it’s Mycroft?
Well, because Mycroft is not everybody. Sherlock believes it when he says that it is not what Mycroft thinks, that is not like last time, that it is for a case, and that he can control it. It’s a quite common belief for people who have struggled with some form of addiction. John, and Molly, and the rest don’t believe him. So what? They are stupid. But Mycroft isn’t, Mycroft is never wrong. Remember, Sherlock gets angry as soon as he realises Mycroft has come to Baker Street. Not a word has been spoken about Magnussen. All Sherlock knows about his brother at this moment is that he is there because of the drugs thing. We aren’t surprised about Mycroft being there. But in Sherlock’s mind he is perfectly capable of controlling it, and if he knows it, so should Mycroft, because Mycroft is never wrong. But if Mycroft thinks that there is cause for concern, that Sherlock could easily relapse, that no case is worth it, that he has been stupid/unwise again; then Sherlock has to consider that he might be mistaken, because Mycroft is never wrong, but he is stupid, at least compared to his brother.
This unwelcome revelation, combined with his distaste for Magnussen and the drugs, might be what caused him to attack his brother. To get him to stop doubting him, and causing him to doubt himself.
Things to note about this particular snapping of Sherlock’s temper:
a) From what he says later, everything about his presence in that drug den was intentional. The entirety of his feigned relationship with Janine was designed to result in information getting back to Magnussen that Sherlock was using again. It was a calculated move to influence the way Magnussen thinks of him. I assume that the fact that he was actually high here was part of his cover; it would be hard to hide out with a bunch of drug addicts without using, yourself. Perhaps he needed something to slow down his thoughts while he was waiting, because he was clearly waiting. We don’t know if he was there every night or just that night, but his absence from 221B was designed to arouse Janine’s curiosity, clearly. Either way, being high did not seem to particularly inhibit Sherlock’s actions here, suggesting that his history of usage made him fairly capable of handling whatever amount was in his blood stream at this point.
b) Sherlock and Mycroft both believe that coincidences rarely exist. Can we really call it a coincidence, then, that John turned up to fetch the neighbour kid from the cot right next to Sherlock’s? Let’s also note that Sherlock revealed himself deliberately. John could well have not noticed that he was even there had Sherlock not attracted his attention.
c) John then refuses point blank to listen to Sherlock’s explanation about why he was there. He jumps to his own conclusions, that somehow in a month of John’s absence from his life, Sherlock broke down and became a drug addict again, completely dismissing Sherlock’s genuine reasons out of hand and treating him the way an angry parent would treat a misbehaving child.
d) He then forces Sherlock into a test, berating him in front of Molly, Mary, and Bill Wiggins, someone who respects and looks up to him. This can’t even be termed an intervention. This is humiliation.
e) Molly then adds her own contribution to the shaming by slapping Sherlock repeatedly and yelling at him. While we can all agree that this would be extremely tempting, this is not how interventions are supposed to be handled. Even if she had yelled at him in private, without an audience, that would have been better. Sherlock apparently more or less keeps his cool here, reverting to sarcasm and barbed quips.
d) John, who hasn’t seen Sherlock in a month, presumably because he was off honeymooning and then enjoying being a newlywed, has dropped Sherlock like a rock, at least temporarily. Sherlock, meanwhile, has set up an elaborate operation targeting one of the world’s least pleasant villains around, a scheme which involved faking an entire relationship, gathering information on Appledore, being hired by Lady Smallwood, setting up an initial meeting with Magnussen to negotiate the return of the Smallwood letters. John now disregards the fact that he’s absented himself from Sherlock’s life and everything that’s been going on in it (none of which John knows yet or tries to find out at this point), and throws it in Sherlock’s face that he could have come to John had he been feeling close to crashing. He’s still on the assumption that Sherlock DID crash, ignoring everything that Sherlock has told him, and conveniently forgetting the fact that he hasn’t exactly been there for Sherlock as a casual friend, never mind in the event of a serious problem. Sherlock does not point this out.
f) John made his feelings about recreational drug usage quite unmistakeable to Sherlock from the very beginning. To have John catch him out AND refuse to believe or even listen to his reasons for it must have been immensely frustrating.
g) Upon returning to Baker Street, Sherlock finds out that John, who has traditionally always sided with Sherlock when it comes to Mycroft, has tattled on him to Mycroft, resulting in an immediate confrontation. Mycroft, being Mycroft, is snide and patronizing. Immediately upon the heels of this seeming-betrayal on John’s part, Sherlock finds out that Mycroft has recruited Anderson, aka his most faithful believer and supporter, to search for a potential stash. Double betrayal. This is the first sign of Sherlock beginning to lose his temper.
h) He then calms himself sufficiently to explain, in deliberate lack of detail, that he is pursuing Magnussen. He does not explain that he has been hired by Lady Smallwood, nor the extent to which his operation is already underway (aka Janine in the bedroom). Mycroft then belittles Sherlock, essentially saying that he has no idea what he’s going up against, that Mycroft specifically doesn’t approve of it, having decided to actively not pursue Magnussen himself, and to desist. He treats Sherlock, just as everyone else in this scene has so far (Bill Wiggins excepted), like a child. A disobedient child who doesn’t know what he’s doing and needs to be put in his place.
So, given that not a single person in Sherlock’s inner circle here a) will give him one shred of credit for what he’s actually doing here, b) are apparently all willing to betray his trust if drugs are the justification for it, c) won’t actually listen to him or engage with him on the same level, and d) are actively shaming and yelling at him to the point of becoming physically violent with him over this, is it any wonder that he finally loses his temper? Sherlock normally manages to repress his frustration with Mycroft in particular to sarcasm and traded snubs, but after the morning he’s had, coupled with the residual influence, I’m not at all surprised that he snapped here. Imagine if his friends and family had, I don’t know, assumed that Sherlock knew what he was doing - which he did, for the record; Janine DID leak precisely what he intended to have leaked; Magnussen had drugs on his mental list of Sherlock’s pressure points (even if he later rejected his theories about every pressure point except for John). I assume that Sherlock intended John to find him there, and after having inquired about it in a civilized manner, was supposed to have been impressed with the depth of Sherlock’s cover and ingenious planning. Instead he got an earful about his so-called relapse and a morning full of everyone close to him reaming him out and invalidating his choices and the reasons behind them. It makes me really angry that not a single person close to him would listen to him for three seconds, or give him ANY benefit of the doubt whatsoever. There are days when I think that no one deserves him, honestly.