Doctor Who: Every Doctor's Darkest Moment

Doctor Who spent years making us believe that regeneration was essentially a rebirth after the previous body died. 

Hell Bent told us that – on Gallifrey, at least – regeneration is just like having man flu. It's a glib aside from the Twelfth Doctor to reassure us that he didn't just gun down the General in cold blood.

Of course, that's the whole point of Hell Bent, which bravely asks the question "What would a Doctor Who finale be like with the Doctor as the big bad?"

The Twelfth Doctor's darkest moment is the culmination of the threats levelled at Me in Face the Raven, and his Time Lord jailers in Heaven Sent. The death of Clara Oswald pushed him over the edge, and made the Doctor turn his back on everything he believed in.