So finally the Doctor ends up in a room with Hitler. The year is 1938. The Doctor or one his companions could kill Hitler. Imagine. You may think you’d never kill anyone, but would you kill Hitler in 1938?
Imagine Poland never invaded. America never liberating Paris. London never going through the Blitz. And millions of people living the whole of their lives.
What would the world be like today? How would Churchill be remembered? What would’ve happened in Russia? Would the Japanese have bombed Pearl Harbor?
How do you imagine the world would be different and where is this parallel universe?
And I want to add…um, Stephen Moffat? He’s writing (as in writing and/or in charge of) two, TWO, shows with iconic British characters. The Doctor and Sherlock Holmes. Imagine being responsible not just for your characters, but famous and indelible and amazing and loved characters. He’s right when he says it is insane. What am I complaining about?
Marta Pelrine-Bacon 
The way I figure is, “Hitler” is being killed/saved every moment. We just never know it.
Like, right now I’m sitting at my desk at home; if I had decided to go for a drive this afternoon, instead, it might have been the trip on which I let my attention wander behind the wheel just long enough, letting the car jump the curb and kill the baby in the stroller who now — thanks to my doing nothing! — will grow up to become a slaughterer of whole populations. You could make yourself crazy thinking like that (although it’s a fun sort of crazy!).
Moffat’s doing a brilliant job. Especially tying together all the little fiddly bits which would be sooo easy to just let a later season’s writers/directors deal with.
Finally watched the episode a little while ago. (When Rose, Martha, and Donna flickered briefly into reality, I have to admit my own heart fluttered a bit.)
Moffat caught me off guard with the images of Rose, Martha, and Donna…well, Moffat catches me off guard a lot. It’s probably why I love him!
Anyway, yes, you can drive yourself mad with that kind of thinking. But if you’re talking time travel, how can you avoid pondering the possibilities. Besides, as writers of fiction, what-if is what we do!
Although I can’t help but remember a scene in a favorite Brit com of mine–Vicar of Dibley. One character says to another, “That’s the worst idea since Hitler’s dad said to Hitler’s mom, ‘Hey, hey, let’s go upstairs, I’m feeling a little saucy tonight.'” Of course, the idea wasn’t as bad as that…
Of course, according to this last Doctor Who episode, there is a justice league giving Hitler hell. If only.