Presumably Flint/Brahms/Akharin/Brack is an Immortal, and can be killed if decapitated. One figures the Highlander folks saw this episode, at any rate, and got some inspiration from it.
Probably the worst case of a decreasing end-of-season (and end-of-series) budget, recycled sets and a minimal Enterprise presence keeps the costs low.
What's disappointing is that Jerome Bixby, who displayed his writing talents for Trek in the second season "Mirror Mirror" seems to have tossed it all away for an exploration of immortality, which he would return to later anyway in "The Man From Earth." It's a well-written exploration as far as it goes, but it fits awkwardly in the Trek universe. It takes a bogus plague (we never see anyone with the disease the entire episode) to create a false sense of drama. It also requires Kirk and McCoy to act wildly out of character: Kirk falling in love instantly and McCoy pouring himself booze when the ship's crew have four hours to live. Even Spock somehow recognizes Brahms' handwriting, and then tampers with Kirk's memories.
There's also just general silliness like the Incredible Shrinking Enterprise.
Only McCoy's speech at the end, and the power of the concept, really pull it out from a 1-2 rating. Daly and Sorrel give good performances and this would have been interesting as a Twilight Zone episode (it doesn't have the "bear" for an Outer Limits story). But here there are just too many contrivances to fit the square peg in the round hole.