For four years, Absalom made many allies in an attempt to make himself king. This is why he acted as a judge for the people. Since this was all done in secret, it is called a "conspiracy." David found out and fled with those in the army who were loyal to him.
The author used the metaphor of a thief to describe how Absalom was able to win people's loyalty: "So Absalom stole the hearts of the men of Israel." (See: [[rc://en/ta/man/translate/figs-metaphor]])