- There was the time they tried to nuke London, which would have killed millions, and some of their plans were designed to try and destroy the world government.
But the threat of the Mysteron attacks has forced humans to develop extremely effective defences and countermeasures. (This is assuming that SPECTRUM isn't the only organization fighting the Mysterons.) These will be needed when the real evil alien invasion turns up. The Mysterons are sinister but ultimately benevolent Well Intentioned Extremists.
- Also, the Mysterons deliberately turned Captain Scarlet into an indestructible man, and then allowed him to go free when he proved that he had Heroic Willpower. They are hoping to produce as many of these supersoldiers as possible because indestructible men will be needed to defend Earth later on. But all of the other indestructible men and women have so far proved disappointing. None except Captain Scarlet have the Heroic Willpower required.
We never see a Mysteron. We see their agents, recreated from objects and people that they've destroyed; but every shot of their complexes, on Mars or on the Moon, is empty of any visible life-forms. We hear "the Voice of the Mysterons" in the original series, but that title only means that it makes "official" pronouncements on their behalf; it might be the A.I. equivalent of a modern American flying a Confederate flag.
It would explain the odd strategy that the Mysterons use. Their stated aim is the destruction of life on Earth. But they concentrate on psychological warfare, which can only affect the sentient life and cannot directly kill anything. They always give their enemies a chance to stop them. They use the minimum number of agents required to accomplish the current job. With their powers, they should be able to crack the planet open without much trouble, and yet they play psychological games. Perhaps they are limited in what "they" can do by previous programming?
Similarly, if a plan is thwarted, they don't keep trying. They may have a back-up plan, but otherwise they move on to a new target. This argues for limited flexibility, such as one might get from a computer. (Remember that this is a 1960s series, and computer programmes weren't as versatile as they are today).
A final point in favour of this argument is Captain Black. He is their primary agent after they killed and recreated him, and their control over him is as complete as possible. Presumably, they know his every thought and memory. Yet they continue their war on Earth, which was started as revenge for his attack on their complex, an attack made in a moment of panic. Even if they didn't bother to find out why this human shot at them, one might expect them to check after Colonel White broadcasts a message to them using a communications crystal taken from their Lunar base which explains that the attack was a misunderstanding and asks for peace. But no, they blow up the crystal in an attempt to destroy Cloudbase. Either their psychology is so alien as to prevent them from understanding human behaviour, or their responses to attack are fixed. Either of those can come from an A.I.
It's confirmed that they're computers in TV 21 (the companion comics)
Captain Black wasn't destroyed and recreated. He was merely inserted into the new timeline by The Mysterons.
- Um, how are you going to explain how at the beginning of the supermarionation series, Captain Black is under his own control?
- Communications blip? If you carried a cell phone to a time before cell towers, it wouldn't matter if the phone still worked otherwise — you couldn't make calls with it. If you took a modern cell phone to a time when all cell phones were analog, you'd probably have the same problem. That's what it would be like: the Mysterons had to reestablish, and maybe retroactively replan, their communications tech, and then take Captain Black over all over again.
Notice how CB speaks just like the "Voice of the Mysterons."
He's always there, even if it's just to tell an agent, "You know what to do."
There are no Mysterons. It's just him. Presumably he found an abandoned alien (not Mysteron, he made up the name) base on a previous expedition, took control of it and, for reasons best known to himself, decided to use it to wage war on mankind.
Or maybe this is his idea of a practical joke.
The whole incident in episode 1 was staged by him as an excuse to declare war. It's all done with recorded messages and remote controlled or programmed devices.
- Helped along by the fact that the Mysterons apparently turn off the control signal keeping their agents alive if they fail their mission or are compromised (e.g by being shot and incapacitated), rendering them vulnerable to normal weapons (Scarlet is the exception in that he can survive without the control signal, because his last thought of fighting to hold on to life was copied into his replicant).
Captain Brown's suicide bombing in the pilot was successful. The Mysterons altered the security recordings to make it look like the President escaped. Note how at the Car Vu, Black tells Scarlet that they need the President alive even though their earlier stated objective was to kill him. Scarlet switching sides was admittedly a Spanner in the Works, but the Mysterons now have an agent at the highest level of Earth government. I wonder when his true mission will be revealed...
For an incredibly advanced race, the Mysterons have a pretty weird mindset. They declare war on Earth due to a misunderstanding in which Captain Black blows their complex up - surely no biggie, given that thanks to the power of retro-metabolism they can instantly rebuild it and anyone in it. Earth poses no threat to them. Earth, in fact, attempts to make peace. The Mysterons seem to have no agenda other than simply mindless destruction. Why?
Think about it this way. If a person is recreated by the Mysterons, it is essentially a perfect replica with the same knowledge and abilities as the original (plus indestructibility or an occasional tendency to explode), except for the fact that it follows the Mysterons' agenda. In a sense, the Mysterons' collective personality is imprinted on top of the replica's own.
Now, let's imagine Mysteron society. An entire city could be destroyed with remarkable ease in the pilot episode. And why not? With fantastic technology like that, Death Is Cheap. Why bother with building to last, with maintenance, even with medicine? It's easy to imagine that for the Mysterons, retro-metabolism could become so universal that basically their entire society could be made up of indestructible replicant-people.
But how did retro-metabolism come about? Well, let's imagine the Mysteron who devised it. Undoubtedly he'd have had to have been a genius, but suppose he was also a total jerkass - vindictive, spiteful and bullying, bordering on psycho. And somehow, when he made his first replicas, his curmudgeonly personality was somehow imprinted into them (we know the Mysterons are telepathic, so maybe that's how). The replicas themselves didn't mind, and the Mysterons as a whole thought it was a small price to pay for creating a post-scarcity society of immortals.
Fast-forward a hundred years. Suddenly you've ended up with an entire society of unfettered assholes. If one of them decides to take offence at an honest mistake, the whole species would conisder it their duty to back him/her/it up at all cost. Any replicants created in the course of this war would also become over-sensitive, vindictive jerks who take Captain Black's mistake personally - aspects not of the Mysteron character, but of the inventor.
Suddenly, the war makes sense. Captain Scarlet's return to the human side demonstrates exactly what remedial action should have been taken, i.e. that the inventor of retro-metabolism deserved a good smack.
Destroying the city after all doesn't mean that much when you have the technology to regenerate the whole thing from scratch. Therefore it makes sense that the Mysterons' revenge seems more like minor screwing around with the humans of Earth- it's just giving them a taste of their own medicine.
The Mysterons' origins were never explained in either the original series or the remake. Therefore, it's conceivable that they were either a race of Shadow-aligned First Ones or the Shadows themselves. Furthermore, the agents they recruited from the Icarus actually became Mysteron duplicates. The only reason Morden was able to be killed was because of the nuke used on Z'Ha'Dum.