Re: TOS set during the 22nd Century!?
The creature pursued its goals by providing each of its victims an image the victim wanted to see. If Crater the man wanted to see his dead wife alive again, so be it - but if Crater the scientist wanted to see a sympathetic monster that was the last of its kind and deserving of nothing but pity, this illusion would also be provided!
For all we know, the Salt Vampire was actually a marooned crewman from the crashed battleship of an advanced starfaring empire, and its chief motivation for infiltrating Kirk's ship was to find a high grade subspace transceiver it could modify into a hyperplexing beacon to call in reinforcements. (Having a snack while waiting for the opportunity would only be natural.)
I think you're overthinking it, Timo
"If we're going to be damned, let's be damned for what we really are." - Jean-Luc Picard, "Encounter at Farpoint"