An insurer publishes misleading brochures that portray its competition as unstable. What best describes this act?

The act of publishing misleading brochures that portray competition as unstable falls under deceptive marketing practices, which are deemed illegal regardless of the circumstances. Insurers are required to maintain high ethical standards and transparency in their advertising to ensure that consumers make informed decisions based on accurate and truthful information. False representations that can mislead potential policyholders or discredit competitors compromise the integrity of the insurance industry and violate regulations governing fair competition and advertising practices.

This aligns with consumer protection laws and insurance regulations that aim to prevent misleading advertising. Such practices can result in penalties for the insurer and damage their reputation in the long run. Hence, the action described in the question is illegal under any circumstances, emphasizing the importance of honesty and accountability in the advertising efforts of insurance companies.