Quoting Commentary for I Samuel 16:14

Now that we have explained [that the verse “Do not be overly righteous or excessively wise” refers to how one is to practice a commandment even if the rationale for that commandment is inapplicable], we [now] need to explain the end of the verse, “Why become demented?” [The explanation is] that this refers to being punished [mentally]; King Saul who, after having been overly kind [to the Amalekites], was troubled with an evil spirit,²³See I Sam. 16:14: “An evil spirit from the Lord troubled him.” and King Solomon who over-philosophized, expelled his knowledge and forgot it.²⁴Midrash Rabbah to Ex. 6:1. This form of punishment lacks a scientific explanation, as it would be unnatural for one who attempts to make himself more righteous or wise, beyond [what is willed by] the Torah, to lose his sanity. However, [this penalty] is a metaphysical punishment decreed with the wisdom of the One who is to be blessed, similar to the way the punishment of kares [is exacted against those] who transgress the thirty-six prohibitions that are punishable by excision.²⁵The thirty-six prohibitions that carry the punishment of excision are enumerated at the beginning of Tractate Kerisos. Therefore, when there are those who sin a hundred times and the Holy One, blessed be He, is patient with them,²⁶Eccl. 8:12. though they are not excised in this world, they will receive a two-fold punishment in the World of Reckoning. Similarly, there are many who think that they are wiser than the Torah in this manner, and they are not punished in this world with madness. Their punishment is being saved for the World of Reckoning. Everything that [we have stated] regarding the explanation of these warnings is provided by the sages of blessed memory. [However, this explanation does not accord with the simple meaning of the verses.]

According to the plain meaning of the text the verse has G’d saying as an assurance to any future King of Israel that he is to fight a war on G’d’s behalf against Amalek. This war would be distinguished by the fact that any loot collected in such a war was out of bounds to any Israelite and would be exclusively G’d’s property. This is why Saul, King of Israel, was punished for allowing the people to retain part of the loot from his campaign against Amalek (compare Samuel I 16, 14-27). This stain on the character of the Israelites (Saul and descendants) continued until the time of Mordechai when the Jews took revenge on their enemies without touching any part of the loot (Esther 9,16). Saul was punished measure for measure. Seeing that the Torah had made the wholeness of G’d’s throne dependent on the war against Amalek being waged by the Israelites, G’d deprived Saul of his own kingdom for failing to heed His instructions to the letter. He did not leave much of a name behind, both he and Yonathan his eldest being slain in battle. Mordechai had appreciated this lesson of history and this is why he exhorted all the Jews that on the day they would confront their enemies and defend their lives they were not to touch any of the possessions of the people against whom they would fight. Haman was a descendant of Amalek. The Torah had warned to “wipe out the memory of Amalek“ (Deut. 25,19), i.e. not to retain anything which ever belonged to that people.
We find that also concerning the battle against Amalek in the future the prophet Micah 4,13 warns: “and you will crush many peoples; you will devote their riches to the Lord, their wealth to the Lord of all the earth.”

MY LORD MOSES, ‘K’LA’EIM’ (SHUT THEM IN). The meaning of this is that Moses had told the people the words of G-d, that He would take of the spirit which was upon him and put it upon those that will stand with him in the Tent of Meeting. Now those that did not go to the Tent [i.e., Eldad and Medad who had also been designated for that honor], and yet held themselves to be among those upon whom Moses’ spirit had been put [by prophesying in the camp,²⁵⁵Verse 27. as Eldad and Medad did], were as rebels against Moses’ words. Therefore Joshua said to Moses, “Shut them in, for perhaps it is a lying spirit²⁵⁶I Kings 22:22. in their mouths, or it is an evil spirit that terrifies them,²⁵⁷See I Samuel 16:14. and thus they must be put in a prison like a man that is mad, and maketh himself like a prophet.”²⁵⁸Jeremiah 29:26. But Moses in his humility answered: “Would that all the people of the Eternal were prophets, that the Eternal would give His spirit upon them!²⁵⁹Verse 29. — for G-d put His spirit directly upon them without taking of the spirit which was upon me, and would that this would happen to all the people.”
However, from the words of our Rabbis it appears that the custom in Israel was that no one would prophesy about future events in the presence of a prophet who is greater than him, but they would follow him as his disciples, and these were the sons of the prophets.²⁶⁰I Kings 20:35. Rambam defines the sons of the prophets as “those who sought the prophetic gift. And although they directed their minds towards it, the Divine Spirit might or might not rest upon them” (Hilchoth Yesodei Hatorah 7:5). Thus the Rabbis have said in Tractate Sanhedrin [with reference to the prophecy of Eldad and Medad]:²⁶¹Sanhedrin 17 a. “We can well understand according to that Sage who said that [Eldad and Medad prophesied that] ‘Moses will die [before entering the Land],’ that Joshua therefore said, My lord Moses, shut them in [since he wished to save Moses from grief]. But according to the Sage who said that [they prophesied] about the quails [that they would come from the sea], why should they be shut in [and not prophesy]? It is because the case resembled that of a pupil who renders a decision in the presence of his master.” Similarly [this was the reason why they were forbidden to prophesy] according to the Sage who says that they prophesied about Gog and Magog.²⁶²Ezekiel 38:17. And the Rabbis have mentioned a similar case in Tractate Megillah in the matter of Deborah.²⁶³In Megillah 14 b the Talmud lists the seven prophetesses that prophesied in Israel, mentioning among them Deborah, Huldah, etc. It is related in the Book of Kings that King Josiah of Judah sent to Huldah a delegation to inquire about a certain matter (II Kings 22:14). On this the Gemara asked: “But how could Huldah prophesy whilst Jeremiah — the leading prophet in Jerusalem — was present?” To this question the answer is given that “Huldah was a relative of Jeremiah and therefore he did not mind her prophesying.” Thus it is clear from this text that there was a rule in Israel that no one should prophesy in the presence of a greater prophet. Ramban’s mentioning “Deborah” is merely a reference to the text in the Gemara which begins this discussion, but the actual reference is to Huldah, as explained above. But Moses said that he, being the master, foregoes the honor due to him [and the law is that a Rabbi is able to renounce the honor due to him],²⁶⁴Kiddushin 32 a. and he desires and is happy [that they prophesy]. And I have seen that the Targum Yerushalmi²⁶⁵This is found in our Targum Yonathan ben Uziel. rendered: “k’la’eim — withhold from them the Holy Spirit.” The Targum thus understood the word k’la’eim to be like the expressions: Behold, ‘lo echla’ (I did not refrain) my lips;²⁶⁶Psalms 40:10. ‘lo thichla’ (Thou wilt not withhold) Thy compassions from me.²⁶⁷Ibid., Verse 12.
Now the reason for this matter is that Joshua thought that [Eldad and Medad] were prophesying because they were of them that were recorded²⁶⁸Verse 26. [among the prescribed number of seventy men that were to join Moses], because the spirit of Moses was transmitted to all that were recorded, for he intended that all [seventy of them should become prophets]. Therefore Joshua said to Moses: “It is not fitting that they [Eldad and Medad] should prophesy in the camp, since they did not fulfill the word of G-d who commanded: and thou [Moses] shalt bring them unto the Tent of Meeting, that they may stand there with thee,²⁶⁹Verse 16. and thus it might appear that not all of them prophesied through His putting of the spirit which was upon you [on them]. It is therefore fitting for my lord to direct your mind towards the spirit of G-d [which is upon you] and to bring it [completely] back to you, so that it should not be taken from you except for those who stand before you in accordance with the word of G-d. For it was because you intended at first that all who were recorded [should be prophets] that some of your glory came upon them, and if you will now intend to withdraw it from them, it will come to rest only upon those who stand before you in accordance with the word of G-d.” But Moses answered [Joshua by saying] that he should not be zealous for his sake, for he wishes that they prophesy whether in his presence or outside it, since G-d had put His spirit upon them — either by transmission from Moses or [directly from G-d] without such a transmission.