וְעָשִׂ֖יתָ כְּחָכְמָתֶ֑ךָ וְלֹֽא־תוֹרֵ֧ד שֵׂיבָת֛וֹ בְּשָׁלֹ֖ם שְׁאֹֽל׃ (ס)
Verfahre also [mit ihm] nach deiner Weisheit, lasse ihn nicht ergraut und in Frieden sterben.
Rashi
And do not let his greying head go down to the grave in peace. Do not let him die a natural death [causing him] to fall into Gehinnom.6Dovid had Yoav’s ultimate welfare in mind.
Tze'enah Ure'enah
“So act in accordance with your wisdom and do not let him descend, etc.” [2:6]. That is, do not kill him until you will find some accusation against Joab. Even though he is guilty of the death penalty, yet nevertheless Joab helped greatly in Israel’s wars. However, he did kill these two. Therefore, kill him wisely through an accusation. In any event, do not let him die in his bed, but let him be killed as he killed.7Radak, I Kings, 2:6.