God's instruction to Samuel marks a dramatic shift in how the Israelites are led, transitioning from miraculous divine oversight to a standard earthly government. God accepts their demand for a king for two reasons. First, the people are no longer worthy of miraculous guidance and must now operate within the natural order [מלבי״ם]. Second, God wants to comfort Samuel, making it clear that the request is not a personal rejection of his leadership [מצודת דוד].
Despite accepting their request, God demands that a severe warning be placed before the people regarding the heavy price of this new government [רש״י, מצודת ציון, ביאור שטיינזלץ]. This caution functions like a formal declaration made in front of witnesses, ensuring the Israelites can never deny being warned in the future [מצודת ציון]. The purpose of this threat is to instill a healthy fear of royal authority [רש״י], which is especially necessary since the nation has not lived under strict rule for hundreds of years [ביאור שטיינזלץ]. Furthermore, it exposes the danger of a king who might ignore Torah law and govern by brute force [מלבי״ם, מצודת דוד].
When detailing the practices of this future king, the primary approach among commentators is that these are not legitimate legal rights granted by the Torah. Instead, they represent arbitrary rules and corrupt habits the king will invent for himself, much like the corrupt practices previously associated with the priests [מלבי״ם, מצודת ציון, רד״ק, ביאור שטיינזלץ]. While an ancient debate considers whether a king is actually permitted by law to take such actions, or if these descriptions are merely meant to frighten the public [רד״ק, אברבנאל], commentators firmly support the latter. Since the Torah explicitly commands a king not to become arrogant or break the commandments, Samuel could not possibly be granting a ruler the legal right to rob and oppress his subjects [אברבנאל].
This creates a bitter irony regarding the concept of justice. The Israelites asked for a king to judge them fairly, but God tells Samuel to reveal what this royal justice will truly look like: the seizure of their property and their own enslavement at the king's whim [אברבנאל]. Ultimately, the warning emphasizes the exact nature of the ruler they are demanding right now—a king who will govern them with absolute, unchecked power [אברבנאל, מלבי״ם].