Radak: It tells the thoughts of the heart, so it is as if it thinks. This is like "uvi'Svunas Kapav Yanchem" (78:72). Even though understanding is in the heart, since the hands guide the flock according to the heart's understanding, the verse attributes understanding to the hands.
Radak: A person intends to cut a little with it, and he cuts much. It is as if the razor schemed. So your mouth said few evil words, and the act that came from them was great; 85 Kohanim were killed, and also children.
3.
Malbim: He intended to cut only the hair and what is repulsive, and it cuts the head. So you intended to remove a pitfall for Beis Sha'ul, and you spilled much innocent blood.
