The phrase "rotten core" means that something or someone appears fine on the outside, but there is a fundamental flaw or corruption within them that makes them bad or not trustworthy. Full definition
It should shock nobody to learn at the very rotten core of the Cambridge Analytica / Facebook scandal lies the unethical use of artificial intelligence. (thenextweb.com)
New contracts for the british rotten core..... (justarsenal.com)
Silent Hill 2 Team Opens Rotten Core in France As Konami nears the end of development for horror sensation Silent Hill 2, the Japanese dev crew shows all in Paris: new shots and interview. (igotaletter.com)