One possibility is that John Hurt is indeed playing the last Doctor; the one who dies. This doesn't mean that he has to be the 12th or 13th Doctor, or whatever, and he could be the 500th. Maybe the 'peace and sanity' he was talking about was his own, and that after so many life-times, and with all of those memories, and painful losses, he decided that he couldn't continue on, and wanted to end it, and so he chose not to regenerate. Since The Doctor seems to have an overwhelming will to live, this act could be considered to be against the basic principles of The Doctor, which is why this incarnation is not deserving of the moniker.
Perhaps not so dramatic as ending his own life, but along the same lines...maybe this isn't about something the Doctor did. Maybe it's about something he didn't do. The Doctor helps people. Maybe John Hurt's Doctor made a decision to NOT step in like he normally would; maybe he allowed an atrocity to happen that he could have otherwise prevented.