I, as I am sure many people do, have a theory on who John Hurt is. I think he's the "real" 9th Doctor. The one who fought in the time war. Here is my reasoning.
1. The Time War is what happened between 8 and 9, and we know the Doctor did terrible things during it (hello genocide). Perhaps this is what he did "not in the name of the Doctor"
2. When we meet nine in the episode, "Rose" he has JUST regenerated (this is evident by the fact at one point he is seen checking himself in the mirror and making passing comments about his new form).
3. During "Journey's End" Ten says to Rose, "Exactly. You were BORN in battle. Full of blood and anger and revenge. Remind you of someone? That's me, when we first met." Indicating that the ninth incarnation was born out of the war and not the one who fought (as in someone before him died during the battle and regenerated into 9).
4. We do not know how Eight died. However 8 is the last incarnation Sarah Jane sees before meeting 10 and when he asks where he went he says, "he had to go to war." So, it is entirely possible he regenerated into John Hurt so he could do what he did in the guise of his real name (and not in the name of the Doctor).