Musar for Bava Metzia 214:10

מצות מינוי המלך . The straightforward meaning of the commandment to appoint a king is to imbue the people with the fear of the authority he wields. They will observe Torah laws better because there is a terrestrial king who will hold them responsible for non-observance. Because of this the king is commanded to be especially scrupulous in his own מצוה observance as it would not do for him to call to order or even convict others for sins which he himself is guilty of. This idea is symbolized by the Torah's command that the king must have two Torah scrolls, i.e. he must be twice as scrupulous as his subjects in observing what is written in the Torah. The reason he is warned not to amass too many wives, too many horses, or too much gold and silver is because each one of these is indicative of too much greed. The possession of too many horses also falls under this heading, since the king must rely on G–d and not on man made means of salvation. We remind ourselves of this in our daily prayers when we recite Psalms 20,8: "They (call) on chariots, they (call) on horses, but we call on the name of the Lord our G–d." Our sages rule that the king must be extra scrupulous in his personal conduct in order to inspire his subjects and not give them an excuse to sin by pointing to his own own lax observance of the commandments; we in our time can apply this ruling to our teachers and rabbis and anyone in a position of authority.