Shmuel Alef Perek 15
In this perek, Shaul is commanded to annihilate the nation of Amalek, even the animals. The Mefarshim explain that the concept that Amalek represented, (doubt and happenstance), hides Hashem's presence in this world. Thus, even the slightest manifestation of this, even their livestock, required destruction. Thus, we do not want anyone even mentioning anything connected to the name of Amalek. Others point out that Amalek possessed some form of magic whereby they could turn themselves into animals). Either way, Shaul goes against this Divine command and has mercy on the animals (not to mention keeping their king alive too) in order to use as sacrifices to Hashem. Shmuel reprimands Shaul for this; notifying him that his reign as king will come to an end. What exactly was Shaul’s sin?
A beautiful idea is put forward by Rav Aharon Lichtenshtein (and others say it too), in a throwback to a comment of the Malbim in Vayera (20:11). Shaul’s mistake was that he went with his own ideas over Hashem’s commands. Shaul felt that it was moral to leave the animals alive, and generally show mercy. What he did not realise (on his level) was that when Hashem commands us to do something, our workings and calculations are irrelevant; especially if these calculations and rationalisations are on the basis of ‘morality.’ For ultimate morality is dictated by Hashem; our versions of morality are subjective and biased - only Hashem’s objective morality is true, real and accurate. And besides, adds Rav Aharon, if Shaul was really going with his own subjective version of morality, why did he kill any of the people of Amalek?! This is also why this sin saw Shaul stripped of the kingship. For a king’s role is to follow Hashem’s will (and lead the people in that direction too) unwaveringly - but Shaul showed that his personal calculations and ideas came before those of Hashem. Such a person cannot be a spiritual leader of the people. This idea is really an offshoot of emunah and bitachon. Just like we rely on Hashem in our lives and events, so too are we to rely on Hashem to dictate what is and isn’t moral.