Heart of King controlled by G-d

 Mishlei (21:1) Like channeled water is the mind of the king in the LORD’s hand;He directs it to whatever He wishes. 

Meshech Chochma (Devarim 17:15): … The Ralbag notes that the heart of the king is in G-d’s hand. Thus the king has no free will but what he does is determined by G d. Shmuel was thus afraid of Shaul at this point because Shaul was no longer king and thus had free will to harm him

Malbim (Mishlei 21:01) King’s heart is in God’s hand Even though a normal man has free will to do as he wishes this is not so for a king. That is because the welfare of the nation is dependent on him. If he choses a harmful action it will produce widespread harm to the nation. Therefore since his actions impact many he is constrained by G-d and has restricted free will 

Or HaChaim (Bereishis 44:18) king's heart is in G-d's hand." This means that the king is privileged to receive divine assistance before pronouncing judgment. 

Rabbeinu Bachya (Devarim 17:15) It would do well for us to study our history and to learn what happened to the Jewish people during the centuries when their political system was headed by a king of flesh and blood. Devarim Rabbah 5,11 sums it up in these words: the Jewish kings caused many of their people to fall in battle because of their faulty policies. Shaul caused many casualties at Gilboah  David caused a plague. Achav, King of Israel, became the cause of the three year famine. Tzidkiyah’s policies became the immediate cause for the destruction of the Temple . How was it that the entire people who had experienced a tremendous renaissance under the leadership of the prophet Samuel agreed to ask for a king? The Talmud Sanhedrin 20 dissects the wording in Samuel I 8, pointing out that the elders of the people were motivated by pure considerations asking that the purpose of the king be “to judge us,”. The common people were motivated by the desire for their king to be a general who would lead them in war, and their sin was in saying “like all the nations.” They spelled this out in greater detail in verse 20 of that chapter. The people were agreed that they wanted a king, but they differed regarding the tasks of that king.

Ralbag (Mishlei 21:01) Heart of a king – This means that the deeds and thoughts of a king are limited by G-d. Because if he had total free will it would pose a great danger to the nation he controlled