That the Daleks' "allies" hadn't realized this late into the game that they were pawns, and pawns with very limited purpose, is a logical flaw in the script. But it does give Stoney a few last moments of increasing megalomania, and a perfectly earned death. One almost expects the Dalek Supreme to add "Shut up, already" as it orders Chen to be exterminated. To the last, he believes himself irreplaceable, immortal. There is even a great moment when Chen tells his fellow prisoners that Kingdom is coming out of loyalty to him, and when Sara confronts him, he promises her an appropriate reward.
Sara Kingdom's death--no; wait. She dies twice, really. First as Kingdom, then as Sara.
Kingdom was, as Ian Fleming described James Bond, "is a blunt instrument wielded by a Government Department. [she] is quiet, hard, ruthless, sardonic, fatalistic."
Kingdom dies when she realizes that her loyalty, which made her the blunt instrument in the hands of Mavic Chen, in whom she utterly believed, led her to gun down her own brother without a pause, without giving him a chance to explain, without a qualm. The instrument had been wielded, Bret Vyon was dead, the mission went on.
Until she found that the mission had been corrupted from the source, and her surrender of her humanity was not for humanity's benefit, but Chen's personal ambition. In Fleming's The Man With the Golden Gun (1965), Bond, who is captured by the Russians after his disappearance at the end of You Only Live Twice (1964), tries to kill M, having been convicted (brainwashed, in his case), that his own surrender of his humanity has been in a false cause.
So perhaps my earlier comparisons of Kingdom with Cathy Gale have been off base; she might better be seen as an inversion of Bond, whose seeming journey from M's "blunt instrument" into "humanity" is a false journey, further into darkness. By contrast, Kingdom's exact parallel journey is one from darkness into light. From the clipped, short tones of Kingdom, to a woman able to laugh, to smile, to see beauty, Kingdom dies and is reborn as Sara. Sara can be stern and tough, but can have friends, and pause to savor those friendships--hence my Cathy Gale analogy.
But then Sara dies. Ironically, she dies the same way Kingdom died--as a direct consequence of disobeying orders, this time the Doctor's orders to go back to the TARDIS with Steven:
DOCTOR: Now, both of you, back to the Tardis.As the episode casts its viewpoint back and forth between the Daleks, Steven watching from the TARDIS, and the Doctor and Sara's desperate effort to escape, Sara's fate unfolds:
STEVEN: What about you, Doctor?
DOCTOR: Do as I say, quickly!
STEVEN: Go on!
(Steven grabs Sara's arm and drags her away. After a short distance, Sara stops and pulls herself free.)
SARA: Wait, Steven. We can't just leave him.
STEVEN: The Doctor knows what he's doing. At least, I think he does.
SARA: All we're doing is running to save our own lives. If anything goes wrong and the Daleks recapture the time destructor, we'll have failed for ever.
STEVEN: I know what you're saying. I'd go back too if I thought it would help. Whatever he's doing, he's doing because he thinks it's the best way. Now come on!
[As he hurries away from the control room, the Doctor meets Sara.)
DOCTOR: What are you doing here?
SARA: I came to help you.
DOCTOR: You must remember, my child, this machine is working. It's working slowly because its range is rather small at the moment, but it is working. Now, if you start to feel strange, you must let me know at once.
SARA: Yes, all right, but what about them?
DOCTOR: First, back to the Tardis. Is Steven not with you?
SARA: No. I came back that way.
DOCTOR: Hurry, my child, hurry!
[Kembel - Jungle]Even with only audio and telesnaps to convey it, Sara's extended aging and death is brutal. We don't know what it that the Doctor says that leads her to shout "You think I don't know?" with a touch of Kingdom's old brusqueness, but the emotion in Marsh's voice is searing. As I read it, it's her response to her own impending death. It hurts to listen to; I can't imagine what it was to watch, with actors like Hartnell, Purves and, especially, Marsh giving it their all.
(Steven reaches the safety of the Tardis and stumbles inside. Still clutching the time destructor, the Doctor leads the way back through the jungle but realises that Sara has fallen behind.)
DOCTOR: It's not far now. Sara!
(Sara is aging before his eyes.)
SARA: Keep going! The Daleks must be after us by now!
DOCTOR: The time destructor is
(His words are lost in the gale force winds)
SARA: You think I don't know?
(The Dalek patrol leaves the underground base and sets off in pursuit of the Doctor. Ahead, the Doctor does his best to help the increasingly infirm Sara struggle onwards, despite the ever-worsening environment.
(Meanwhile, Steven is pacing impatiently up and down. Not sure what else to do, he turns on the scanner.)
(Feeling helpless, Steven takes out his frustration on the Tardis console.)
[Outside the Tardis]
(The Daleks maintain their dogged pursuit. All around them the jungle is changing, plants withering and dying, lush foliage shrivelling up and turning to dust. Apparently unmoved and unaffected by the vast power of the time destructor, the Daleks continue to close in on their quarry. Yet the effects are taking a heavy toll on both Sara and the Doctor. Frail and weak, the old man stumbles onwards, all too aware that time is quite literally running out for his companion. At last they see the Tardis ahead but the Doctor can barely stagger a few more steps before he falls to his knees unable to go on. With a final desperate effort, Sara helps the Doctor up and they stumble a few short paces further before the Doctor falls again. The time destructor slips from his fingers, rolling away to lie just inches from his hand. Beside him, Sara pitches forward in the dust, unable to remain upright. She barely has the strength to open her eyes but somehow finds the will. She attempts to claw her way over to the time destructor.)
(The last of her strength gone, Sara falls and lies still.)
She dies, because she couldn't let the old man face the danger alone. She dies, because she can't risk failure, and leaving the universe vulnerable. Sara gets her killed as much as Kingdom does. More so, really, because she could have watched from the safety of the TARDIS-the Time Destructor is wreaking havoc on the Daleks too. So she dies because she can't let a friend take all the risk without sharing it.
It's a dark ending, to a dark, and long, story. The Doctor tries hard to draw some meaning from it all, to even attempt a weak Merlyn's laugh. But Steven is having none of it, and, ultimately, the Doctor agrees:
STEVEN: I wish Sara could have seen the end.The series has never left us in so grim a place before. Where do we go from here?
DOCTOR: Yes, my boy, so do I. You know, Steven, the one thing that Sara lived for was to see the total destruction of the Daleks. Well, now it's all over. Without her help, this could never have been achieved.
(The Doctor notices something in the sand and reaches down to scoop it up in his hand.)
STEVEN: What is it?
DOCTOR: Millions of years of progress reversed back.
(The Doctor opens his hand to show Steven a tiny dead embryo.)
DOCTOR: That's all that remains of a Dalek.
STEVEN: Let's go, Doctor. I've seen enough of this place.
DOCTOR: Well, my boy, we finally rid this planet of Daleks.
STEVEN: Bret, Katarina, Sara.
DOCTOR: What a waste. What a terrible waste.