I think the big question isn't how bad etc Harry is or gets, but will he be aware that he is? If he isn't how does he discover that he is? Or in the biggest twist in history, he remains the villain for the rest of the series....
In the original Star Trek story, yes, there were nasty mirrors for Kirk and Company, but the Kirk and Company we know and love were wandering around in the parallel Enterprise, shocked but aware and trying to fix things and get back to their own universe.. Meanwhile Spock and Company also became aware rather quickly that the Kirk and Company that showed up were not the good guys we know and love, locked them up and began to figure out a way to send them back and get their old Captain and Company back...
Sooooo..... Based on that, since the story is told in first person, we won't suddenly have Harry run amuck as the new Kemmler... He won't think he is doing anything wrong, and there won't be anyone to tell him that this isn't him... Because in their world, big bad Harry is normal! I think it will go down like the original Trek, Harry shows up in the mirror world and discovers big bad Harry and either tries to change him or kill him... Or he finds someone like Spock with integrity in both worlds to kill him, most likely this will be Murphy, Michael, or Eb...
So one can speculate, but as I said, years off, find it hard to get excited, that is an opinion and not stopping anyone else who wants to speculate...