According to her story, is her son Chayav Misah?
Radak (7): No, for there were no witnesses or warning - "v'Ein Matzil Beineihem." He is liable bi'Ydei Shamayim, but not at the hands of Beis Din. Malbim - we learn this from "ba'Sadeh". There are another two reasons to exempt. (a) There was no one to save. Had there been someone to save, it would not have come to this. It was close to Shogeg, amidst the heated anger without a mediator. (b) The victim was like a Rodef (refer to 14:6:2:2). All three of these apply also to Avshalom. There were no witnesses or warning, there was no one to save - had David judged Amnon for raping Tamar, this would not have happened, and also Amnon 'hit' Avshalom, via raping Tamar. 1 Mahari says close to this.
How does this make him like a Rodef? If one chases a single woman to rape her, we may not kill him to save her (Sanhedrin 73a)! The entire comparison is difficult. Avshalom commanded others to kill Amnon; he himself did not strike him! And what is the source that there were no witnesses? Bnei ha'Melech saw something that made them flee! (PF)
Why does it say "va'Yako" with the suffix Vov?
Radak: [He struck him. Even though it says later "Es ha'Echad",] sometimes the pronoun precedes the noun, e.g. "va'Yshanoe Es Ta'amo" (1:21:14), "v'Zihamato Chayaso Lachem" (Iyov 33:20).
Malbim: The superfluous pronoun is as if the verb is repeated. This is like "va'Tir'ehu Es ha'Yeled" (Shemos 2:6) - she saw the Shechinah and she saw the child. Here, each one hit the other, so the victim was like a Rodef.
