Why did the verse specify the mothers of those who killed him?
'Rashi', Rashi (Melachim II, 22:1) from Mechilta Beshalach, Amalek 1): It teaches that Hashem paid (punished) him through people like himself. Amonim and Mo'avim are ungrateful - they denied the good that Avraham did for Lot (from whom they descend) - he fought the four kings to save Lot, and they 1 hired Bil'am to curse Avraham's seed. Sons of women from these nations will pay Yo'ash, who denied the good that Yehoyada did for him, and killed his son Zecharyah.
The Torah mentions only Mo'av hiring Bil'am. Almost all Meforshim explain that Amon may not marry into Yisrael due to not offering us food when we passed by their land; also this shows ingratitude (PF). Malbim (Devarim 23:4) - at the time, Melech Mo'av was king also over Amon; it is as if also they hired Bil'am.
