This is a spiffy cool idea. *swipe* Now it's part of my campaign. 
Now, as for more ideas...the failed alchemist is a great choice for your villain, maybe gearing up toward lich-dom after the final killing.
I'm harsh, so I wouldn't let the players get him before the last one, but make him pay in the days after (like they get to the person just in time to see him fall to the bbeg), which starts a great final fight as he may have gained powers after that killing.
Of course, he could be a ringmaster or circus owner travelling with a bunch of kobold entertainers killing someone in each town.

Now, as for more ideas...the failed alchemist is a great choice for your villain, maybe gearing up toward lich-dom after the final killing.
I'm harsh, so I wouldn't let the players get him before the last one, but make him pay in the days after (like they get to the person just in time to see him fall to the bbeg), which starts a great final fight as he may have gained powers after that killing.
Of course, he could be a ringmaster or circus owner travelling with a bunch of kobold entertainers killing someone in each town.
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