Captain Scarlett is said be indestructible altho almost towards the end of episode he performs an act of courage that causes him to die, but that’s ok because he will always regenerate.
What is this regeneration? Regeneration comes from the Mysterons. Mysterons are some myterious creatures of which we never see their appearance, they only manifest themselves by using destructive, copying and regenerative beams which they blast through the earth’s atmosphere.
Here is the key to why Captain Scarlet is permanently regenerating; the earth’s atmosphere. The atmosphere is what prevents the Mysterons from having a permanent hold on Captain Scarlets mind. When they killed the original captain scarlet and replaced him with a copy the Mysterons made an error getting the dosage right on the regenerative beam, either giving him the inherent ability to regenerate after death, which was only meant to be for one use only. Or making a mistake with the control beam, and it not being strong enough to control an indestructible agent after the first death.
The second theory could explain why Captain Black is so insidious and never really puts himself in harms way; were he to die he would become another indestructible asset for humanity.
Following their failure with creating an indestructible agent (or failure to regenerate Captain Scarlet for single use), the Mysterons have always used hit and run attacks. Often similar to acts of terrorism.
This leads me the believe that the powerful beams that the Mysterons use are severely restricted by the earth’s atmosphere, as when the Mysterons were first introduced they could literally blast everything to shreds, but not from the distance of the moon to earth, this is why they only seems to focus on destroying and regenerating one target per episode.
My conclusion is that there are two heros in Captain Scarlet, The Captain and the Earth’s Atmosphere.