Final answer:
The developer in the case study with the 'anti-worm' likely violated the principle of responsible computing under the ACM Code of Ethics.
Step-by-step explanation:
In the case study where an "anti-worm" was released to combat the effects of another worm, it's indicated that the developer violated the ACM Code of Ethics. Specifically, the violation most likely pertains to responsible computing. This principle falls under the category of ethics within the field of software engineering, which is governed by codes such as the IEEE-CS/ACM Joint Task Force on Software Engineering Ethics and Professional Practice. Releasing such an 'anti-worm' into the wild can have unintended consequences, such as damage to unintended targets, which would not align with the ethical obligations of software engineers to ensure their products do not diminish quality of life, impinge on privacy, or harm the environment.