There have been a lot of news articles lately about psychopaths and the test that can supposedly sort the psychopaths from the garden-variety criminals. So here’s a related Poirot quote. He once said,
It is an interesting subject of after-dinner conversation—are all criminals really madmen? There may be a malformation in their little grey cells—yes, it is very likely. That, it is the affair of the doctor. For me—I have different work to perform. I have the innocent to think of, not the guilty—the victim, not the criminal.
from Peril at End House