My mind keeps wondering over what we know about John Hurt's character. Out there are some interesting theories, but theories that don't seem to fit the facts. What we will find out in the 50th Anniversary Special in November remains to be seen, but here are my guesses.
John Hurt did what he did "without choice - in the name of peace and sanity." He is the Doctor's secret and did not do what he did as the Doctor. Clara saw 11 faces, 11 Doctors. What could this all mean. She jumped into his time stream at his tomb. She should have seen all the Doctors, from the first to his death, but she did not see past the Eleventh or before the first. Now, the Doctor had a life before he became the Doctor. He had a family and a granddaughter, Susan. We know that there will be a twelfth Doctor (Peter Capaldi) and long ago, at the end of Trial of a Timelord, it was hinted there would be a thirteenth and something pulled from that regeneration that we knew of as the Valyard.
The two key points to my way of thinking are the timestream and the Twelfth incarnation. Clara saw 1-11, and if there was something buried in the middle, she would have seen it. But there was before and after as well. I do not think that John Hurt is playing a part that fits anywhere between An Unearthly Child and The Name of the Doctor. I strongly feel it must be before, after, or (a remote possibility) something to the side (like the Valyard). I think the 50th Anniversary is key to what the event is. I think that a celebration of 50 years is going to be key to the story. I'm strongly leaning to John Hurt being the Doctor before he took that name. A different incarnation? Possible, but doubtful.
Now, we have a couple of examples of side Doctors. The 10th nearly regenerated, but instead ended up with a clone from his severed hand. The Valyard, a mysterious amalgamation of the Doctor, also, was a separate being. Both are technically the Doctor, but not. Perhaps there is another and instead of the secret of why he took the name of Doctor and ran, it might be a mysterious amalgamation from between his Eighth and Ninth incarnations, a tool used in the Time War. That would also harken back to the Doctor's past.
The other possibility is that we are looking toward the future. Perhaps John Hurt is the last Doctor and the terrible thing he did was what we saw on Trenzalor.
Still, looking a the event and what might be possible in the Doctor Who universe, I still am leaning toward a pre-Doctor first Doctor and an event that made him run. But we need to wait until November to find out.