The episode began with John helping a neighbor find her son, who is a drug addict. Of course, he finds her son. What he doesn’t expect is to find Sherlock lying on the floor next to him. Because Sherlock has a long history of drug problems, no one believes him that he did it for a case. Even, the mild-mannered Molly Hooper smacks Sherlock across the face. Sherlock wasn’t looking for a high, at least not off of whatever drug he was using, he was luring Charles Augustus Magnussen onto his trail. Given Sherlock has a flair for the dramatic, it makes sense that Sherlock would pretend to resort to old habits just to get to the “one man he truly hates.”
Magnussen is basically Rupert Murdoch with a huge mansion and a malicious mind palace. Sherlock may like to call himself a sociopath, but Magnussen is one. Magnussen makes a living out of blackmailing people and publishing the results. Not even Sherlock is immune from Magnussen’s power. Granted, Sherlock gets himself into trouble and always manages to make it out okay, but Magnussen did have Sherlock good. He manipulated Sherlock via his pressure point: John. Specifically, Magnussen threatens to disrupt John’s life as he currently knows it. He can reveal Mary Watson’s darkest secrets- secrets she hasn’t told John because she loves him and doesn’t want to scare him.