1)
Who fled, and why?
1.
Rashi citing Targum Yonasan: The Jews in Be'eros, near the Pelishtim, fled after Sha'ul died (1:31:7). 1
2.
Radak (2): The two officers fled; it does not say why. Perhaps they had a quarrel with Sha'ul or his son, and now they returned. They made peace with him beforehand, therefore the household did not beware of them.
3.
Radak: According to Menachem, that the officers killed a Ben Sha'ul (not Ish Boshes), this is why they fled.
4.
Malbim: The officers had killed a man, therefore they fled. They returned only today, that they came to kill Ish Boshes.