For the ‘wickedness’ of demanding a king, during the wheat harvest Samuel calls on Yahweh to send thunder and rain; which he duly does. This comes at the culmination of a speech in which the seer condemns the worship of the Baals and Astartes — the gods of the land, and it is interesting in that Ba'al is the Canaanite god of kingship, fertility, and weather. The worship of Yahweh alone is clearly a later interpolation. However, it seems entirely plausible that this addiction is rather a revision of an identification of Yahweh with Ba'al. It is, after all, Yahweh who is called upon at the Canaanite high place (bamah) at Gilgal during the wheat harvest to send rain — a disaster for the crop. As an interplay between Yahweh and Ba'al, this text is clearly related to the contest between Elijah and the prophets of Ba'al (1 Kings 18:20-40). The wheat harvest is a time of rejoicing. This, coupled with the sacrifices and feast to celebrate the kingship of Saul, is a great feast which Samuel has turned into mourning. Samuel has quite actually rained on their parade.
(יז) הֲל֤וֹא קְצִיר־חִטִּים֙ הַיּ֔וֹם אֶקְרָא֙ אֶל־ה' וְיִתֵּ֥ן קֹל֖וֹת וּמָטָ֑ר וּדְע֣וּ וּרְא֗וּ כִּֽי־רָעַתְכֶ֤ם רַבָּה֙ אֲשֶׁ֤ר עֲשִׂיתֶם֙ בְּעֵינֵ֣י ה' לִשְׁא֥וֹל לָכֶ֖ם מֶֽלֶךְ׃ {ס} (יח) וַיִּקְרָ֤א שְׁמוּאֵל֙ אֶל־ה' וַיִּתֵּ֧ן ה' קֹלֹ֥ת וּמָטָ֖ר בַּיּ֣וֹם הַה֑וּא וַיִּירָ֨א כׇל־הָעָ֥ם מְאֹ֛ד אֶת־ה' וְאֶת־שְׁמוּאֵֽל׃
(17) It is the season of the wheat harvest. I will pray to the LORD and He will send thunder and rain; then you will take thought and realize what a wicked thing you did in the sight of the LORD when you asked for a king.” (18) Samuel prayed to the LORD, and the LORD sent thunder and rain that day, and the people stood in awe of the LORD and of Samuel.