John Harrison isn't the villain, he's just some hapless Starfleet officer who was killed by Lord Garth, the shapeshifter, and replaced so that he could carry out his nefarious schemes to insanely destroy the Federation bwahahaha!!! Cumby is playing two characters.
Which leaves open the question, what does Lord Garth look like in reality? He needs to be an older guy - is that who Weller is playing?
And that's my final answer, I'm sick of coming here and feeling like a damn ping pong ball!