Radak: The writing is Nif'al. Even if you do not give them, Hashem will give (punish them for us). The pronunciation is Pa'al - you should give them.

Yerushalmi Sanhedrin 6:7: Sha'ul killed seven Giv'onim when he killed Kohanei Nov.

Radak (1): There were seven survivors from Beis Sha'ul of those who killed the Giv'onim. Refer to 21:7:2:3.

Malbim: There were only seven survivors from Beis Sha'ul, excluding from Yehonason. They knew that Yehonason did not consent to eradicate Nov, for he had a Bris with David.

Rashi: We will hang them for the sake of Hashem, to publicize His judgment (Radak - that He wants to avenge what was done to us).

Radak: This way, all Yisrael will see their vengeance, and be careful not to harm Giv'onim again. Malbim - they will learn from a Kal v'Chomer. If so was done to the king's sons for harming us, all the more so others [who harm us will be punished harshly]! They did not seek vengeance, just to fix the matter so they will not be pursued any more. ¹

Malbim: Bnei Binyamin and Sha'ul's family live there; they are drawn after his mindset. When they see the men hung in Giv'as Sha'ul, they will fear and not sin b'Mezid again.

Rashi citing Brachos 12b: A Bas Kol (voice from Heaven) said so. Radak - i.e. his sins were pardoned.

Radak: They said it; Hashem chose him to be king.

Radak (1): It is better to uproot a Parshah in the Torah than to have Chilul Hashem. I.e. the Nesi'im had sworn not to kill them [and they killed them]! Also refer to 21:6:8:1.

Radak (1) #2: Beis Din does not kill children for the sins of their fathers This punishment was bi'Ydei Shamayim.

Malbim (from Bamidbar Rabah 8:4): This was unlike Torah law. The Torah says that we do not kill two people the same day ¹ , and not children for the sins of their fathers, and to hang the bodies on the tree [after the day of death]. Only a king may do so ² , like the Rambam says (Hilchos Melachim 3:10).