Sforno on Exodus 20:5/ספורנו על שמות כ׳:ה׳

1לא תשתחוה להם, to any phenomena in the universe not man-made.

2א-ל קנא, I cannot tolerate that someone who worships Me worships also someone beside Me. The reason is that there is absolutely no comparison between Me and any other phenomenon in the universe. I am therefore entitled to stand on My dignity by refusing to be compared.

3פוקד עון אבות, the reason why I sometimes wait so long before punishing some of the wicked in this life is that by waiting and noticing that their offspring, i.e. second and third generation continue in their wicked ways their measure of guilt will become so full that I will exterminate that whole family from life in this world. [this is what happened to the family of Jerobam for instance, and that of Omri, his son Achav, etc. ] This was possible only by adding the unrequited sins of the fathers to those of the sons, or even subsequent generations, so that when finally, retribution did come, it wiped out the family completely. This happened in the third generation of the original sinner such as Omri’s grandchildren. It also happened to the fourth generation after Yehu (who himself had been G’d’s instrument for wiping out the family of Omri/Achav) when his great grandson Zecharyah was murdered after reigning only 6 months. (Kings II 15,9) We know about this method G’d employs sometimes already since Genesis chapter 15 when G’d explained to Avraham that his descendants could not expect to take possession of the land of Canaan until the fourth generation, as the sins of the Emorite would not be irrevocable until then. (Genesis 15,16)