What is the meaning of "v'Ratzu Lifnei Merkavto"?
Radak: The horsemen will run in front of his chariot.
Radak, based on Targum Yonasan: Men will run on foot in front of his chariot. Malbim - this is a lowly Avodah.
Why did he mention these matters?
Radak (9, citing R. Yosi Sanhedrin 20b): The king is truly entitled to all of this.
Radak (9, citing R. Yehudah Sanhedrin 20b): The king is not entitled to this. He said so only to scare them.
Malbim (9) - Mishpat ha'Meluchah (10:25) is laws that the Torah fixed for kings - another may not ride on his horse... Here, they requested a king not bound by Torah law. He gave examples of laws that he may make according to his whim, to show that it will be bad for them, unlike they thought!
