Albert Barnes Commentary 1 Samuel 31:10

"And they put his armor in the house of the Ashtaroth; and they fastened his body to the wall of Beth-shan." — 1 Samuel 31:10 (ASV)

In the house of Ashtaroth - This was undoubtedly the famous temple of Venus in Ashkelon, which Herodotus mentioned as the most ancient of all her temples. This is why Ashkelon is specifically mentioned in 2 Samuel 1:20. Placing Saul’s armor as a trophy in the temple of Ashtaroth was a counterpart to placing Goliath’s sword in the tabernacle (1 Samuel 21:9).

In 1 Chronicles 10:10, it is added that they fastened Saul’s head in the temple of Dagon, likely in either Gaza (Judges 16:21) or Ashdod (1 Samuel 5:1–3). This was perhaps in retaliation for the similar treatment of Goliath’s head (1 Samuel 17:54).

These variations seem to imply that both this narrative and the account in 1 Chronicles 10:1–14 were compiled from a common and fuller document.