What was the moral conclusion of 'The Doomsday Machine' ?
Something that would've been more evident to viewers during the Cold War than to viewers today. The term "Doomsday Machine" was coined by a US strategist in the 1950s. To quote Wikipedia
RAND strategist Herman Kahn proposed a "Doomsday Machine" in the 1950s that would consist of a computer linked to a stockpile of hydrogen bombs, programmed to detonate them all and bathe the planet in nuclear fallout at the signal of an impending nuclear attack from another nation. The key aspect of the doomsday device's deterrent factor is that it would go off automatically without human aid and despite human intervention, providing a highly credible threat that would dissuade attackers and avoid the dangerous game of brinkmanship that brought the United States and the Soviet Union closer to nuclear war during the Cuban Missile Crisis. With a doomsday device on the planet, neither side would suspect the other of launching a sneak attack in attempt to destroy the opposing country's infrastructure before they could retaliate.
This concept showed up in a lot of fiction over the next few decades, most prominently in Dr. Strangelove
. It was part of the fear of global destruction that people lived with every day. And I'd say the moral of the Trek episode of that name was that a doomsday machine was a terrible idea as a society's only hope of avoiding cataclysmic war -- that it was really just a continuation or culmination of the drive to destruction rather than an antidote for it. In short, the planet-killer was an allegory for the bomb. Kirk even explicitly said so in dialogue, complete with a big melodramatic music sting to drive it home to the audience.
Granted, "war is bad" is not a very complicated moral. But frankly the superficiality of "The Doomsday Machine" is why I think it's an overrated episode, and not a good exemplar of TOS's intelligence as a whole.