Final answer:
The aggressive personality change during chemotherapy is most likely due to medication side effects, which can affect mood and behavior.
Step-by-step explanation:
The change in behavior from a mild-mannered personality to a more aggressive personality during chemotherapy can be attributed to multiple factors. However, the most likely reason for this change is medication side effects. Chemotherapy drugs are known to affect not only cancer cells but also rapidly dividing healthy cells. This collateral damage can lead to a range of physiological and psychological side effects. These side effects can include changes to mood and personality due to the considerable stress the body is undergoing. Additionally, the emotional strain of dealing with a serious illness can exacerbate these changes. Therefore, while psychological effects, biological changes, and emotional stress can contribute, the direct impact of chemotherapy medication on the body is most likely the primary cause.