A historical villain had ordinary mornings that explain nothing away.

People who did terrible things also ate breakfast, worried about their health, and were fond of somebody. Knowing that is uncomfortable, and it is more accurate than treating them as a separate category of creature. Nothing in the ordinariness excuses anything. What it does is remove the comfortable idea that harm at scale requires a monster rather than a person with reasons.

Hold both facts at once rather than choosing the easier one. It is the more demanding position and it is the only one that tells you anything useful about how such things happen. Recognising the ordinary in it is what makes vigilance possible.