The term often used to describe someone who uses mental illness as an excuse for their bad behavior is "mental illness malingering" or simply "malingering." Malingering refers to the act of intentionally exaggerating or feigning symptoms of a mental or physical condition for personal gain or to avoid responsibility for one's actions.
It's important to note that mental illness malingering is a complex issue, and not all individuals who claim to have mental health problems are necessarily malingering. Some people may genuinely struggle with mental health issues and need support and understanding. However, when someone uses their mental health as a way to manipulate or avoid accountability for their actions, it can be harmful and perpetuate stigma surrounding mental health. Professional evaluation and diagnosis by mental health experts are essential to differentiate between genuine mental health challenges and malingering.