Comment by sz

On the other hand, if someone has been lying all their life, if they're ever going to tell the truth, it's going to be on their deathbed.

This idea actually appears in our legal system: in court, hearsay (Person X testifies that they heard Person Y say something about Event Z) is usually rejected because there's no way of knowing whether Person Y was lying, even if Person X was truthful. But there are exceptions for circumstances under which Person Y is considered likely to be telling the truth. One of them is a dying declaration; statements made on a deathbed.