They brought him on horses. — Rather, they carried him upon the horses — i.e., perhaps in the royal chariot wherein he had fled from Jerusalem. Or, perhaps, the corpse was literally carried on horseback by the regicides.
The orderly method of proceeding, the burial of the king in the royal sepulchres, and the elevation of Azariah, seem to prove that the murder of Amaziah was not an act of private blood-revenge.