Radak

I will not be afraid of ten thousands of the people: – for all Israel were with Absalom, except those who were with David in Jerusalem, and had gone out with him, and those who had joined him afterwards. That have set themselves (שתו) against me round about: – They have set their battle in array against me. Or the interpretation of שתו may be “they have fought.” And so “they have taken their stand” (fought, שת שתו) “at the gate” (Is. 22:7); and “He will assail (ישית) thy waves with violence” (job 38:1 1).