Then when that happens that might be morally objectionable. But probably like any other weapon that already exists, a rocket, missile or gun.

While not everyday a new defense systems is invented that is targeted at statistical weapon that terrorizes civilians.

In Batman Begins, the villian just makes the drinking water toxic. With todays AI and Biotech, one can create a new bacteria or virus and cripple water supply of cities. I am sure a suitable trained AI can get more creative with such low cost attack vectors.