When we find out about the true abilities of the beast, all bets are suddenly off. The knife could have been physically held by Hengist, Scotty, even McCoy for the seance killing, with amnesia or feelings of vague dread replacing the personal knowledge of the deed committed. People having memory blanks could be stabbers themselves, or patsies for the crime, or just further victims that the beast wants to see wriggle with fear.
It's pretty clear the writers didn't think through all the implications, but also that they didn't put much effort into establishing exactly who did what; in the end, the beast was behind it all anyway, and basically nobody else was to blame.
But as for the monster originating on Earth, again that seems unlikely simply considering the sheer precedent. The beast arrived on Argelius with a space traveler. Argelius may be an ideal starport, but it doesn't strike me as any more starfaring itself than 19th century Earth. Hengist may have been recognized as a man from outer space, even when the person who hosted the beast's Jack the Ripper tour was mistaken for a man or woman from Earth, but that's just details.
I guess the interesting question either way is the numbers of Redjacs out there. Is every mass murderer one (or the victim of one)? How many Redjacs per planet at any given time? Do they always play it petty, or do they also have political ambitions, igniting global wars for extra terror? Or, at the other end, do they first direct horror movies and then inhabit the projectionist for a snack?
It's not as if killing sprees of the sort described would be a particularly rare or special phenomenon ITRW, or in Trek (DS9 and VOY gave us a modern serial killer each, and TNG had several). Yet something prompted Spock to single out two specific sprees on Earth. What makes those Redjacky? And are the rest "natural", or simply the work of a fellow beast with slightly different tastes?
Timo Saloniemi