Why did he mention Yonason?
Malbim: Even though he was a tool of the salvation [if he sinned, he will die].
Why should he die?
Ramban (Vayikra 27:29), cited partially in Malbim: "Kol Cherem Asher Yacharam? Mos Yumas" teaches that the king or Great Sanhedrin has the power to impose a Cherem, and one who transgresses is Chayav Misah 1 . Also Achan and Anshei Yavesh Gil'ad were Chayav Misah for transgressing the Cheramim of Yehoshua (Yehoshua Perek 7) and the Tzibur (Shoftim Perek 21), respectively.
Malbim: According to the Hashgachah that clung to him, this increases the sin that he did to arouse Hashem's anger. The closer one is to Hashem, the greater is his sin. 2
