Malbim: Avah is in the heart; Mi'un is verbal. Initially he did not want [to cease chasing him] in his heart. Now that Avner said 'why should I kill you and cast you to the ground?', he feared, and in his heart he wanted to turn away. However, his mouth refused, for it would be shameful [to turn away] due to Avner's threat.

Radak: Some spears, at the end of the wood it is sharp, and a sharp iron (is attached) to insert it in the ground, like it says about Sha'ul "va'Chaniso Me'uchah va'Aretz" (1:26:7). Sometimes one strikes with it backwards - he does not turn to face his pursuer to strike him with the blade, rather, with the back. So Avner did.

Rashi citing Sanhedrin 49a: It is the fifth rib; the gall bladder and liver are attached to it (therefore it killed him).

Rashi citing Targum Yonasan: The side of his thigh. ¹

Sanhedrin 49a: No. He was able to (aim for and) strike the fifth rib. All the more so he could have struck a limb that would not kill him!

Malbim: Yes. Asah'el was a Rodef (pursuer). Do not say that he could have stopped him by maiming a limb. He struck him with the back of the spear. One cannot aim so precisely with the back as when striking with the blade. He did not see in back of himself; by chance, he struck the fifth rib. ¹

Rashi: He died in that place. Radak - this is like "v'Lo Kamu Ish mi'Tachtav" (Shemos 10:23). I.e. he did not move after he was hit; he died immediately.

Radak: When David's men heard that Asah'el was killed, they chased after Avner. When they reached the place where Asah'el died, they stopped. Yo'av and Avishai did not stop; they pursued Avner [Malbim - for they were Go'alei ha'Dam].