Oh, for god's sake, it's all a matter of taste and the degree of ''seriousness' or 'childishness' in any given story is entirely in the eye of the beholder.
This story was a classic example of fictional morality, where, the writer being God, he can make the benign moral choice (the one the writer is espousing) turn out to be the correct one, because he controls the fictional reality.
Real drama, proper drama, does not do this; it holds a mirror to the real world, and shows us what it is like. Real drama does not provide a cop-out for the hero. And a cop-out in this case is anything other than two things happening: the Doctor giving whatever the guy's name was over to the cyborg or the cyborg destroying the town. This story was as much fairytale as anything you're likely to see. The Doctor magically finding another way - that is the stuff of childish fantasy. That is why I liked last week's episode: the Doctor had a decision to make and he made it. The writer didn't give him a get out of jail free card, nor did he feel the need to have characters talk in pompous (and out of character) speeches about the moral choices they were being presented with.
Anyone who thinks this latest episode made the slightest useful comment or observation about real world morality is childish, in my view. And see, that's the point: it's all relative to one's own mindset. So can we stop this kind of crap about whether episodes have a suitable gravitas or sobriety to them and judge them on the terms of what they were actually trying to be?
My judgement of this episode: it was just too banal for me.