The Sma"g explain the rationale of the פטור of כל זרעו [that if one gives all of his seed to the Molech he is פטור] that when Beis Din executes someone, the person receives atonement [כפרה]. When a person gives all of his children to the Molech, the sin is so egregious that Hashem doesn't want this person to receive atonement.
Apparently that makes no sense - you can have a person who gives his children and grandchildren to Molech 100 times [!!] and he will be חייב and receive כפרה. While if someone has ONE SON he will be פטור as Tosfos says [if he is an only child] and receive no כפרה. Who has done the worse sin? OF COURSE the person who sins 100 times. So according to the logic of the Sma"g, why does the worse sinner receive a כפרה while the lesser sinner doesn't?? Like - so COUNTER-INTUITIVE!!
We see from this as we said earlier - The severity of the sin is not in the מעשה of passing his seed through the fire for the molech per se but rather in sacrificing the GENERATIONS to the Molech. Hence, if he passes over even one child, it will be more serious than passing through 100 children, provided that this one child is his only seed. That being the case he has given over ALL of his progeny to the Molech and therein lies the seriousness of the infraction.
