Even the best of us have certain psychological mechanisms that can suddenly kick in and turn us into monsters.
Neal StephensonJust as an informal, nonscientific observation, most people's personalities don't seem to change very much during their lives. There are exceptions in the case of people who go through hugely traumatic events or suffer from brain injury or disease. Some would argue that religious conversions can have deep personality-altering effects. But these are all exceptions to the rule.
Neal Stephenson