But plenty of people cannot stomach voting for poisoning the townspeople in the first place.

But they are not doing anything against that by abstaining from voting. They are still giving their consent to the poisoning, just by doing nothing instead of doing something, that is literally the only difference.

My whole point is that the “inaction is better than action” bias when evaluating options is bizarre to me.

Edit: https://en.wikipedia.org/wiki/Omission_bias seems to be the term used for the phenomenon.