How, David realizes, do the people mistake his mercy for fear?

Absalom and Achitophel is a political allegory on events in Restoration England. The character of King David is meant to represent Charles II, who ruled England between 1660 and 1685. In Dryden's epic poem the Jews are presented, like late seventeenth-century Englishmen, as being stubborn and self-willed. As such, they are profoundly dissatisfied with David as their king, despite his mild, benevolent rule. They constantly plot against him, believing that they have the God-given right to overthrow a monarch who doesn't meet with their approval. Dryden is making a satirical point here, criticizing those of his contemporaries who took advantage of Charles's good nature and benevolence to undermine his rule.
One by one, the people David thought were his friends and allies start to desert him. The people are on the brink of revolt, convinced that David's mercy is a sign of weakness. Revolution's in the air and it seems like only a matter of time before the king is violently overthrown. Those remaining loyal to David such as Hushai and Adriel advise him that he must act firmly if he's to hang on to his throne. No more Mr. Nice Guy; David needs to assert his kingly authority and fast.
https://www.poetryfoundation.org/poems/44172/absalom-and-achitophel