I remember someone (or maybe even multiple people) claim that “Battlefield” wasn’t so much about racism as it was about how unceasing hatred can led to destruction, as in the case of the people of the planet Cheron and eventually Bele and Lokai.
“Doomsday” was always one of my favorite episodes. I agree with you said about Windom.
They never explored the original of the planet killer in any other Star Trek show or movie, but in a ST:TNG novel called “Vendetta,” the doomsday machine was portrayed as a weapon made to fight the Borg. The novel was about a more advanced model going after the Borg (a few months after the events of the Best of Both Worlds two parter).
Out of the third season, I also enjoyed “The Tholian Web,” “The Enterprise Incident,” and “Day of the Dove.”
Re: “Battlefield”, that is also another way of looking at it. It was rather a fitting end to the episode that all Kirk & Co. could do was shake their heads and walk (or warp) away. It reminds me a bit of the deranged/unhinged hatred that is at the core of the left. Simply no way to reason with people that lost.
Re: The Planet Killer. That might’ve made for a good movie with the right script, although I’m not sure I’d agree with the notion that it was created to fight the Borg. It was too much a malevolent entity. The Borg wished to assimilate all, whereas it simply destroyed anything in its path for energy, so that’s an even worse concept.
The 3rd season did have some good eps. Lack of funds forced them to become more creative, and that’s often a good thing.