So Bad Wolf Watched.
Bad Wolf watched the TARDIS, and saw her born among the singing of her sisters, and saw her fall in love with a young Time Lord, and saw her steal him away, and saw her live, and saw her die among the singing of her sisters.
Bad Wolf watched Jack, and saw him live before the Doctor showed him how to Live, and woke him up from Death, and saw him found Torchwood, and saw him fall for Ianto Jones, and saw Ianto Jones fall, and saw him run, and saw him die, and saw him run, and saw him die, and saw him run, and saw him live, and saw him die as the Face of Boe.
Bad Wolf watched the Doctor, and saw him born on a planet with an orange sky, and saw him lose a friend at school, and saw him at the birth of his child, and saw him at the birth of his grandchild, and saw him fall in love with a blue box, and saw him yell at his granddaughter when he realised she had stolen away with him, and saw him leave her behind when she fell in love, and saw the others come aboard and leave him, and saw him leaving Sarah Jane, and saw him leaving Rose Tyler, and saw him leaving Amy Pond, and saw River Song, and saw the Doctor die for the last time.
Bad Wolf watched Rose, and saw what had been expected, but also saw other worlds. A world where the Doctor was human, and married Rose, and their children were beautiful and blessed. A world without stars, where the TARDIS was dying and Rose was saved from a human boy by the a Nestene drone who was more human than a human had the right to be, and guarded the Pandorica until the Doctor could return the world to how it was meant to be.
Bad Wolf watched Rory Williams, and saw Amy Pond tell him about the Doctor, and saw Melody Jones (who would be River Song) tell him about Rose Tyler and the boy called Jack, and saw the Doctor invite him inside the TARDIS, and saw him die in a tunnel, and saw the claws of nothing reach out for him. And Bad Wolf screamed, because Rory had not saved Rose.
So Bad Wolf Acted.
Bad Wolf grabbed Rory, not the body but the essential Him that was being leeched away, and stole him away from the nothing, and held him close until there was a tugging at the parts of him that were already gone, and followed them to the Nestene Consciousness, which was building a column of the Roman Army, and found the one that was Rory, and settled the essential Him inside of it.
Then Bad Wolf watched Rory Williams fight against the Nestene Consciousness, and punch the Doctor, and threaten Caesar, and pull the Pandorica across the English Channel, and take Mass, and yell at the Pope, and pull the Pandorica across the Channel again, and kick King Henry VII in the shin, and punch a few Jesuits, and teach Shakespeare how to play football, and curse at Napoleon, and teach a girl how to dress like a boy and go to war, and write down a blinded man's poems, and shoot down a V1 missile, and punch a Nazi in the face, and save Rose Tyler, and shoot a Dalek with his hand.
And then the stars came back, and Bad Wolf could breathe again.
But Rory Williams kept dying, and he hadn't lived out his time yet, and now Bad Wolf cared to see it lived out. So Bad Wolf pulled Rory Williams back from the brink of death when he drowned, long enough for the siren to save him. And Bad Wolf pulled Rory Williams out of the sniper's line of fire, and blinded the sniper. And Bad Wolf kept Nephew in the right place when his mind was filled with death to Rory, and ensured his freedom from House. And Bad Wolf cradled Rory's head when Jennifer punched him. And Bad Wolf pulled Rory to Amy's memory when she'd been left by herself for thirty-six years. And Bad Wolf would have saved him from the creatures that had had him killed before...
...but Amy Pond saved him first.
And Bad Wolf was pleased, because some other time or place was calling, and it was time to move on.