Let his children be fatherless - Hebrew, “his sons.” This is what “always” occurs when a criminal who is a father is executed. It is one of the consequences of crime; and if the officer of justice does his duty, of course, the sons of such a man “must” be made fatherless. The prayer is, simply, that justice may be done, and all this is but an enumeration of what must follow from the proper execution of the laws.
And his wife a widow - This implies no malice against the wife, but may be consistent with the most tender compassion for her sufferings. It is simply one of the consequences which must follow from the punishment of a bad man. The enumeration of these things shows the enormity of the crime - just as the consequences which follow from the execution of a murderer are an illustration of the divine sense of the evil of the offence.