1)
What Chachmah should he use to kill him?
1.
Radak: Do not kill him until you find a [new] sin 1 . Even though he was Chayav Misah [for killing Avner and Amasa], he fought wars for Hashem, and had merits. Therefore, bring him to a [new] reason to be Chayav Misah.
2.
Malbim: Show why there is a need to kill him [via a king's power, unlike Torah law] for a need at the time.
2)
What is the meaning of "v'Lo Sored Seivaso b'Shalom She'ol"?
1.
Rashi: Do not let him die [naturally] by himself and fall to Gehinom. Radak - rather, kill him, like he killed.
2.
Malbim: You need not kill him, but you must punish him in a way that he will not die amidst serenity.
