Sherlock lays out his reasoning on CAM’s information as he goes through the show. It goes as follows: (spoilers)
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1: When Sherlock tells John about CAM, he explains his understanding that CAM keeps vast vaults of information under his house which he uses for blackmail purposes. He states that he believes CAM does not store information on a computer or network, because such things can potentially be hacked. (Where he got the information that led him to this conclusion, I don’t know. Possibly CAM himself has deliberately obfuscated the issue, since at the end he does seem to be aware of what Sherlock means by ‘vaults.’)
2: CAM comes to Baker Street, looks calculatingly at Sherlock, and says ‘Redbeard.’ Having just watched CAM size him up, Sherlock realizes from this that CAM must somehow have access to his information even outside his house. Thus, he must NOT store his data only on hard copy.
3: Sherlock then infers (incorrectly, as it turns out) that CAM does indeed have a database. He also puts together CAM’s ‘dead-eyed’ unfocused look with the possibility that he is using a technology like Google Glass to access this database on the fly.
4: To test this, Sherlock invites CAM to pay him a visit at the hospital. There, Sherlock takes his glasses to check them out—but is dismayed to discover that his theory was wrong. They are perfectly normal glasses.
5: Sherlock is forced to return to his original premise that CAM has his information stored in his house. (Why he doesn’t think of a memory palace, I’m not sure, but hey—we all know that “there’s always something.”)
Sherlock tells us outright why he loathes CAM so much: he’s a disgusting shark of a man who preys on people using their differences as weapons against them. He says this twice, in fact—the first time when he’s explaining to John, and the second time when he’s trying to get Mycroft to take his side. (And he’s offended when Mycroft doesn’t seem to care about that argument.)
Furthermore I think there’s a moral repugnance there. Sherlock cares about some peoples’ lives—he tells John to take care of Janine—but not other peoples’ lives—he dismisses the security guard as a member of organized crime and a child molester.
And Sherlock is quite right, this is the core of blackmail: if you dare to veer from a generic mass ‘normality,’ then in the wrong hands, that deviancy can be used against you. This is effective to a greater or lesser degree, based partly on the illegality of your activities and partly how heavily you yourself weigh the impact of that deviancy. For example, some of us in fandom would go to great lengths (hopefully not illegal ones) to hide our fannish activity. Others are perfectly content to publicly link to our porn-ridden fan Tumblrs. For some of us, this is a matter of our own perception. If our secret were exposed, it may or may not have any noticeable effect on our lives. For others, it’s a matter of real danger, such as the possibility that one’s boss might fire you if they knew.
Mycroft’s “Go against me…” threat is obviously an attempt to keep Sherlock from investigating CAM. There could be two reasons for this. Mycroft’s job is to keep the UK on an even keel, so the first possibility is that he doesn’t want CAM stirred up as too many public officials could be caught in the crossfire and it could destabilize the security and smooth function of the country. The second possibility is that Mycroft is aware that Sherlock himself is Mycroft’s weak point, and he fears that if Sherlock tangles with CAM, in the end it could expose Mycroft to CAM’s manipulations—and that could have internationally destabilizing implications, as Mycroft not only serves in a uniquely delicate capacity for the UK but also sometimes outsources his talents to allied nations.
Of course, these two possibilities are not mutually exclusive, so the answer could very well be ‘both.’
