What is the meaning of "Lo Chen Ochilah Lefanecha"?
Pirkei d'R. Eliezer (52): The man did not want to tell Yo'av where Avshalom is. Yo'av bowed to him, and [when he still refused] forced him, and he showed the place to him.
Rashi: I will not ask you further. (I will go to do so myself.)
Radak: It is not good that I hope for you and speak to you [to kill him. I will do so myself.]
Malbim: Do not strike him. I will begin to strike him; the sin will be mine. Indeed, he only began; his youths finished his death. 1
Did Yo'av fathom that three spears through his heart would not kill him (refer to 18:14:4:1*)?! Perhaps these words were like a Nevu'ah (unknown to him) that he would only begin his death. (PF)
What are "Shevatim"?
Radak: They are sticks with a blade at the end, like spears; the stick is called Shevet.
Why was Avshalom's heart pierced with three spears?
Sotah 9b: It is because he deceived three hearts - of his father, Beis Din and of Yisrael. Radak - 'Beis Din' refers to the 200 men who accompanied him (15:11) unaware of his plan. They were all heads of Sanhedriyos.
What is the meaning of "Odenu Chai"?
Radak: He was still alive after the three spears passed through his heart 1 . This is why the youths struck him (verse 15), until they killed him.
What is the meaning of "b'Lev ha'Elah"?
Radak: The spears passed through his heart, and were inserted in the tree. This is like "Akeh v'David uva'Kir", "Akenu Na ba'Chanis uva'Aretz" (Shmuel I, 18:11, 26:5). Lev ha'Elah is the middle of the tree, like "Ad Lev ha'Shamayim" (Devarim 4:11).