There is an important subtlety which is that engineers are not responsible for which technical challenges get solved, they are responsible for how those technical challenges are solved. An engineer's ethical obligation is not to avoid working on something that could be used in a harmful way, it is to prevent incidental harm from its deliberate usage. There's nothing unethical about working on a fuel additive to prevent engine knock; knowingly using a poisonous compound which is going to create immense public health problems to solve the problem was the sin.