1)
Why did David cut off their hands and feet?
1.
Malbim: Their hands murdered, and their legs went to inform David.
2)
Why did he hang their bodies by the lake?
1.
Radak: It was so everyone would see, and know that David in the straightness of his heart took vengeance for Beis Sha'ul, even though they were his enemies.
2.
Malbim: It was so no one will be Mezid to strike the king. David did so based on his status of king; it is not Torah law.
