Malbim: The Magid did not kill Sha'ul. Even if he finished his death, it was in war, and Sha'ul was already Chayav Misah [for his Aveiros, like Shmuel said via the Ov]. Here, they killed Ish Boshes in his house, not in war, and he was a Tzadik (not Chayav Misah - all the more so) I will seek (avenge) his blood and eradicate you [murderers of innocent people]!