Why did Gid'on tell his son to kill them?
Malbim: His son was Go'el ha'Dam; the kings killed his uncles. 1
Perhaps Gid'on distanced himself from taking a large share of the spoils (refer to 8:24:1:1), but he did not want to deprive his son. If the son kills them, he will receive what was on them [ornaments and precious garments]. (PF)
Gid'on was a closer relative (the brother)! Surely, the Mitzvah or Heter for Go'el ha'Dam to kill a Yisrael murderer applies only to the closest relative. R. Akiva (Makos 2:6) holds that anyone else is liable for killing him! Here, Malbim justifies why Gid'on asked his son to kill them, for also this is like death via Go'el ha'Dam. (PF)
What was Yeser's fear?
Rashi: He was afraid of the kings. Metzudas David - he did not understand that they are handed over to him, and will not stand against him.
