The king of ancient Israel was not an absolute monarch. He had to obey the laws of the Torah. The Torah includes both religious laws and 'secular' laws.
The ministers of the king were not allowed to listen to the king if he commanded to break any laws.
However, the king had certain rights that the average citizen did not have, such as being allowed to create a thoroughfare through private property.
The king could be tried by the sanhedrin, the supreme court of Israel. This right originally applied to all kings of Israel, but was limited to kings of the house of David when King Yanai of the house of Hasmoneans intimidated the sanhedrin.
The king had certain laws that he had to follow over and above the rest of the nation, such as having to always have a Torah scroll with him, and having limits on the amount of money and horses he was allowed to have.
The Jewish hope for a Messiah is that a king arise from the house of David who while ruling over the people of Israel, will follow the laws of the Torah and build its institutions.
