What is a mistake? Is it something that results from a miscalculation? Maybe a broken promise? It could be something we've done wrong.
But, what is wrong? To us, wrong is the undesirable handle put on to something we've done bad. When we've made a mistake.
But to these "wrong" people, what they do is good. Or they delude themselves into thinking it is good. Or they decide it is not really bad or that it won't hurt anyone.
Do some know they are evil? Do they want the handle of "wrong"?
Everyone makes mistakes. Even that man in a blue box that flies around time and space, saving people.
Some are saved. Some this man cannot save.
Some say, "He saved me from death!"
Others say, "He didn't have to let those people die."
And, "He saved everyone but me."
Does this man look back at all the mistakes he has made? No one really knows. To him, his mistakes are a monster of the past that he never defeated, his greatest foe. Not Daleks, Cybermen, or even the Master. The shadow of all his past mistakes is his greatest foe, an evil shadow of the past. One that is always there, always following close behind him. It is ready to accompany him when he saves a world from destruction. He saved that planet, but the mistakes still remain.
He can't really defeat it. It'll always come back.
What about a mistake being a broken promise, or a broken hope? Hope is the result of someone making a promise to another. If the promise was broken, the hope would also. Hope and promise, broken together.
When the man looks back on all his mistakes, he thinks about the hope he gave to people and the promises he made.
"I promise to get you home safe."
"Don't worry, no one will die today."
"You are safe."
What happens if those promises are broken?
She died trying to get back to her home planet of which she was told, of which she was promised, was safe from the Daleks.
Everyone died in that colony ship.
He was never safe from the Cybermen.
This man is afraid to look behind himself and fight the monster, the shadow of the past.
He thinks about all the friends he knew that died traveling with him.
Adric. C'rizz. Sara. Katarina.
But, there are good things too. That promise he kept. The life he saved. The friend who he showed the universe to and whisked back home in time for tea.
Even in those good things, the bad things still remain. He can't atone for them. He wanders the universe trying to trick himself into thinking the good things he does destroy the bad. But, they don't.