Why did he separate the nation from his son and himself?
Rashi: It was so the lottery will determine among which group was the sin. Radak - one parchment in the lottery said Kol Yisrael, and the other said Yonason and Sha'ul.
QUESTIONS ON RASHI
Rashi writes that the lottery will determine whether the sin was among Yisrael, or among Yonason and Sha'ul. It is not normal to cast a lottery like this!
Abarvenel #1: The one who told Yonason about the Shevu'ah, he told Sha'ul covertly that Yonason ate. Perhaps he anticipated that the people will redeem Yonason, and he wanted to showed Midas Chasidus in front of them.
Abarvenel #2: If Sha'ul did not know that Yonason ate, he wanted to show that he equates himself and his son to the smallest Yisrael. If the lot will fall on him and his son, he will do Mishpat [to the sinner]. 1
Perhaps Sha'ul was concerned lest people say like Achan, that a lottery must fall on one of the people, even if all are innocent (refer to Yehoshua 7:19:1:1). In a normal 'chance' lottery without Hashgachah, the probability that it would fall on Sha'ul or Yonason is miniscule; Sha'ul seeks a scapegoat, and is willing to take a tiny risk. To refute this, Sha'ul made the lottery so that if it were by chance, the chance that it will fall on himself or Yonason is 50%! This showed absolute confidence that via Hashgachah, the lottery will identify the sinner! (PF) Perhaps this is Abarvenel's intent (refer to 14:40:2:2).
Also if he cast a regular lottery, when it falls on Yehonason, they will see that he is ready to kill him! However, this way, they see that from the beginning he was aware of this possibility. (PF)
