Aggression is a key issue in a forensic unit. To be able to prevent or manage aggression, we need to understand its underlying dynamics. The present study explores how psychosis, personality disorders (PD) and psychopathy may differ in the motivational dimensions of aggression, and how motivation is affected by a limit-setting situation. Staff assessments of aggression motivation were collected between 2006 and 2015 and comprised 2100 incidents, involving 47 patients. Multilevel analysis was used to predict instrumental, defensive and irritable motivational dimensions of aggression at individual and incident level. Levels of motivational dimensions of aggression varied across diagnoses and psychopathy traits. A limit-setting situation was associated with an increased level of all motivations for aggression. Diagnostic characteristics also affected the impact of limit-setting situations differently; psychosis was associated with an additional increase in instrumental aggression, while higher levels of the antisocial factor of psychopathy were associated with an increase in irritable aggression. To help increase the understanding and management of aggression, attention should be directed towards factors such as diagnostic traits, motivational dimensions for aggression and the patient’s own perspective.