Samuel stands before the people to deliver a powerful visual lesson about the gravity of their demand for a human king. By manipulating the forces of nature, he exposes the stark contrast between the desires of the people and the direct providence of God.
The event takes place during the wheat harvest, which falls in the middle of summer around the festival of Shavuot. During this season, the weather is hot and the skies are completely clear of clouds to allow for the harvest [רלב״ג, ביאור שטיינזלץ]. Against this backdrop, Samuel warns that his prayer will instantly summon violent storms, complete with lightning and thunder [ביאור שטיינזלץ]. The primary approach among commentators is that rain during the harvest is not a blessing, but rather a curse that brings ruin and disaster. Although God does not wish to cause harm without reason, He alters the natural order and answers Samuel's prayer to deliver a sharp message to the people and to validate His prophet [רש״י, מצודת דוד, ביאור שטיינזלץ].
The specific choice of this miracle is not random; it perfectly mirrors the nature of their request for a king. One perspective suggests that just as God brings down destructive rain simply because Samuel asked for it, He also agreed to give the people a king merely because they demanded one, even though it will ultimately harm them [מצודת דוד, אברבנאל]. Another approach focuses on the issue of timing. Rain is fundamentally a blessing for the world, but when it falls out of season, it becomes a curse. In the same way, while a king might be beneficial under different circumstances, demanding one now—while Samuel is still alive and God is protecting them directly—is destructive [מלבי״ם]. Furthermore, the historical timing is premature, as no worthy candidate from the tribe of Judah, the tribe destined for royalty, is yet ready to rule [חומת אנך].
Because of this, Samuel declares that their wickedness is great [מצודת ציון]. The sudden storm proves that God's miraculous providence still actively accompanies them, meaning there was never a need to ask for a mortal king, bound by the laws of nature, to save them [מלבי״ם]. Moreover, the miracle exposes their deep disrespect for Samuel himself. If the sheer power of his prayer can summon a fierce summer storm, he could have easily brought thunder down upon their Philistine enemies to save them, just as he had done in the past, entirely eliminating the need for a king [רש״י, מלבי״ם].
Ultimately, the wickedness of their demand is defined by how it is viewed in the eyes of God. The people cannot yet see the long-term damage that will result from establishing a monarchy. However, God, Who looks ahead at all future generations, clearly sees the ruin and harm that will grow from this decision. Therefore, even though God officially granted their request, their action remains deeply harmful and contrary to His divine providence [מצודת דוד, אברבנאל, מלבי״ם].