How is Chachmah better than weapons?
Rashi: The Chachmah of Serach - "va'Savo ha'Ishah El Kol ha'Am b'Chachmasah" (Shmuel II, 20:22) helped more than all the weapons they had to fight against Yo'av 1 .
Ibn Ezra: Chachmah does not need weapons. It already said "Tovah Chachmah mi'Gevurah" (16). Chachmah saves its 'owner', and others with it, e.g. people of the small city.
Rashbam: The poor Chacham's Chachmah was better than those who besieged the city.
Rid: Chachmah helps more in war and everything, more than all weapons that will be there.
R. Avigdor: Yakov's Chachmah was better than all Esav's weapons. Yakov armed his household with weapons inside, and garments outside - "va'Yachatz
Why does it say that one sinner destroys much good?
Rashi citing Koheles Rabah: Had [Serach] not killed 1 Sheva ben Bichri, all would have perished due to him.
Torah Temimah citing Kidushin 40b: One should view himself as if his merits and sins weigh equally. If he does one Mitzvah, he is now mostly a Tzadik. If he does one sin, he is mostly evil!
Rashi (from Kidushin 40b): If Yisrael were half Tzadikim and half Resha'im, and a person came and sinned (and made himself a Rasha), he tilted the judgment for everyone to guilt.
Ibn Ezra: This refers to "Moshel ba'Kesilim" (17).
Rashbam: Just like a poor Chacham saved the city with his Chachmah, so one sinner ruins much good. Ri Kara - the coming verse explains how.
Rid: One Choteh (omission) can cause the war to be lost.
R. Avigdor (based on Koheles Rabah): This refers to Esav - "va'Yashav ba'Yom ha'Hu
I.e. she convinced the people of her city to kill him; it says "va'Yichresu Es Rosh
