We discuss the early insertion of formal analyses in distributed malware defense evaluation, and provide an example method for applying an executable rewriting logic specification to drive both simulation and property validation of a collaborative group-based worm defense. An important aspect of the algorithm under consideration is its distributed and probabilistic nature, which makes the defense system harder to attack but unfortunately also complicates the ability of designers to fully understand its behavioral properties. We demonstrate one approach to formally analyze our case study worm defense algorithm, employing tools that facilitate both statistical simulation and property validation. Our approach is posed as complementary to the current practice of informal design specification and evaluation through network simulation.