Research on violence perpetrated by individuals with major mental illness (MMI) typically focuses on the presence of specific psychotic symptoms near the time of the violent act. This approach does not distinguish whether symptoms actually motivate the violence or were merely present at the material time. It also does not consider the possibility that non–illness-related factors (e.g., anger, substance use), or multiple motivations, may have been operative in driving violence. The failure to make these distinctions clouds our ability to understand the origins of violence in people with MMI, to accurately assess risk and criminal responsibility, and to appropriately target interventions to reduce and manage risk. This study describes the development of a new coding instrument designed to assess motivations for violence and offending among individuals with MMI, and reports on the scheme’s interrater reliability. Using 72 psychiatric reports which had been submitted to the court to assist in determining criminal responsibility, we found that independent raters were able to assess different motivational influences for violence with a satisfactory degree of consistency. More than three-quarters (79.2%) of the sample were judged to have committed an act of violence as a primary result of illness, whereas 20.8% were deemed to have offended as a result of illness in conjunction with other non–illness-based motivating influences. Current findings have relevance for clarifying the rate of illness-driven violence among psychiatric patients, as well as legal and clinical issues related to violence risk and criminal responsibility more broadly.