This post may contain spoilers for those poor souls who haven't made it to the end of Season 3. Be ye warned.
While the Sherlock universe is not without its villains, Sherlock's oldest foe is more diabolical and shadowy than Moriarty himself.
I speak, of course, of Mycroft Holmes.
At first blush, Mycroft seems like an okay guy. He works for the government, protecting the British people from disaster. He roots out terrorists, sweeps away scandal and is at the heart of every crisis before it even happens.
So why is he the villain?
Elementary, really. It all goes back to their childhood. Burdened by parents who could never quite understand them, Mycroft was the only person that could've been considered Sherlock's equal. He likely had more influence over Sherlock than any other human being on the planet. Yet his frigidity and condescension served only to mold his little brother into the cold, disdainful loner we all know and love.
Even after they've grown up, Mycroft's scorn for "ordinary" people is only surpassed by his contempt for Sherlock. No matter what incredible feat of deduction Sherlock may perform, Mycroft is always distant and sardonic. Without the validation Sherlock unknowingly craves from his brother, he eschews all human intimacy, alienating himself from others and even acquiring destructive habits.
In short, Mycroft shaped Sherlock into the man he is today.
Is this a bad thing?
Some would argue no. The relentless competition between Sherlock and Mycroft may have been the catalyst for the detective's impressive powers. Without it, Sherlock may never have lived up to his full potential. It's certainly a question for the ages.
Is there hope for Mycroft?
Mycroft remarks several times throughout the series that "middle age comes to us all". Maybe this is causing him to realize the effect of his treatment of Sherlock. When he watched Sherlock shoot Magnussen, the regret in his eyes alluded to more than horror at the murder witnessed. Maybe, for the first time, he truly saw the person Sherlock had become and blamed himself. Does that mean Mycroft will find reconciliation and peace? Guess we'll have to tune in to Season 4 to find out.