When the Israelites along the valley and those on the other side of the Jordan saw that the army of Israel had fled and that Saul and his sons had died, they abandoned their cities and ran away. So the Philistines came and occupied their cities.
The day of recompence has now come, in which Saul must account for the blood of the Amalekites which he had sinfully spared, and that of the priests which he had more sinfully spilt; that of David too, which he would have spilt, must come into the account. Now his day has come to fall, as David foresaw, when he should descend into battle and perish, Sa1 26:10.
Commenting on 1 Samuel 31:1-7
And which the men of Israel that were on the other side of the valley,.... The valley of Jezreel; of which See Gill on Hos 1:5, and they that were on the other side Jordan; or rather "on that side"; for the phrase will bear to be rendered either way, and so may mean that side of Jordan on which the battle was fought; for...
the men of Israel that were on the other side of the valley--probably the valley of Jezreel--the largest and southernmost of the valleys that run between Little Hermon and the ridges of the Gilboa range direct into the Jordan valley.