Or HaChaim on Deuteronomy 7:10/אור החיים על דברים ז׳:י׳

1 ומשלם לשונאיו, "and He repays them that hate Him." Why is the first half of this verse written in the plural, i.e "those who hate Him," whereas the individual who is the victim of this repayment is described in the singular? Perhaps the meaning is that unless G'd foresees that not only the individual who hates Him has sinned but also his children and children's children will continue to sin, He would forego repayment, (retribution), awaiting the penitence of the sinner's children. If, however, G'd foresees that the sinner begets only other sinners, He has no incentive to delay repaying such a sinner. This conforms to the principle ברא מזכה אבא, that "a son may confer a merit on his father."

2The verse concludes with the singular, i.e. ישלם לו, "He will repay him," as the Torah speaks about individual sinners and the word לשונאיו was the exception. The Torah adds לא יאחר, "He will not delay." This means that if G'd deals with only a single שונא, person who hates Him, one whose children will not relate to G'd with hatred, then He will devise a means of punishing that person in this life immediately so as to prevent him from incriminating himself further if he were to commit still more sins while being allowed to live. By punishing such a person promptly either by death or afflictions, G'd demonstrates His love as He has hopes that the son of the sinner will confer merits on him through his exemplary conduct on earth.

3The meaning of the words אל פניו, "to his face," is to be understood in connection with Jerusalem Talmud Avodah Zarah chapter 3 that shortly before their deaths G'd lets the righteous experience a foretaste of what is in store for them after death in order that they should die happily. The reverse is true of the wicked. G'd shows them a foretaste of what is in store for them so as to make their deaths a painful experience for them (compare Esther Rabbah on Esther 1,12 that Queen Vashti refused the king's command). The Torah adds: "He will not delay repaying him who hates Him to his face." This refers to punishment in this life; before the face of the sinner is brought to burial G'd will show him what is in store for him. He will not wait until the sinner gets to Gehinom.