Poisoning the water supply is different because the water supply is managed by the local government, and because it's a crime. These add a personal risk to the decision, which your argument is indirectly relying on when you assume it's obviously a bad thing.

You should choose an example that wouldn't lead the executives being personally attacked by prosecutors:

* An oil spill killing ocean wildlife and damaging the look of a beautiful beach.

* A soda company deciding whether to put 10% more sugar in their sodas to increase sales, even though they know its likely to kill statistical thousands of people.

* A manufacturing company moving to China, pressuring local workers into working long hours in poor working conditions and polluting their country.

I believe none of these is a crime, and I wouldn't argue that someone who voted against these should resign from the board rather than help execute on a decision they oppose.

Do you have a better example?