Why did David think that Saul would come to destroy the whole city?
1 Samuel 23:10 (ESV)
10Then David said, “O LORD, the God of Israel, your servant has surely heard that Saul seeks to come to Keilah, to destroy the city on my account.
Given the fact that Saul ordered the destruction of the priests and all their families in the town of Nob, David assumed that the same would have happened to the men and women of Keilah if he should have remained with them.
10 Then David said, “O LORD, the God of Israel, your servant has surely heard that Saul seeks to come to Keilah, to destroy the city on my account.