This study examined how feeling anger and the tendency to attribute hostility vary developmentally depending on whether malicious intent was clearly expressed or not before the harm occurred, and whether the harm was caused by commission or omission. Considering hostile attributions for harmful acts is ethically important in coping with aggressive behavior. In moral and legal contexts, harmful acts are generally classified into two types: commission (doing something) and omission (doing nothing). Previous studies have shown that, from an early age, people tend to judge acts of commission as morally worse than equivalent omissions (this is called omission bias). Japanese fifth graders (10-11 years), seventh graders (12-13 years), ninth graders (14-15 years), and university students participated in this study. Each participant read four scenarios combining two levels of malice (clear or unclear) and two types of acts (commission or omission). In each scenario, a classmate first did a transgression to the protagonist and said something either clearly malicious (e.g., "You're such a fool not to have avoided this") or unclearly malicious (e.g., "I'm sorry, I didn't realize"). Then, another harmful event for the protagonist was caused by the classmate's act of commission or omission. Participants were required to rate how hostile the classmate's intentions appeared and how angry the protagonist would feel. The results showed that hostile attribution and anger ratings were significantly higher when malice was clearly expressed and when the harm was an act of commission, and these tendencies became clearer with age. These findings are consistent with the previous finding of omission bias and indicate that both children and adults are more likely to perceive intentional, active harm as more blameworthy. This study also suggests that perceptions of intent and the nature of acts are connected to ethical judgment and emotional responses across development.