Hashem said to Moshe, "Say to the Kohanim, the sons of Aharon..."
The Midrash (ילקוט שמעוני רמז תרכו) expounds on this Passuk as follows.
מלמד שהראה הקב"ה למשה, דור דור ודורשיו, דור דור ושופטיו, דור דור ומלכיו, והראהו את שאול ואת בניו נופלים בחרב. אמר לפניו, רבש"ע, מלך ראשון שיעמוד על בניך ידקר בחרב. אמר לו הקב"ה, ולי אתה אןמר, אמור אל הכהנים שהרג שמקריגין אותו.
Hashem showed Moshe all of the future Leaders, Judges and Kings that He will appoint over the Jewish Nation. Hashem also showed Moshe how King Shaul and his children will fall to the enemy's sword. Moshe told Hashem, "The first King that You anoint over Your children shall be slain with the enemy's sword?" Hashem responded, "To me you say this? 'Say to the Kohanim' whom he killed, and who accuse him". [Ed note: We need to realize that King Shaul was of the greatest of our People, and it is beyond our comprehension to understand why he felt it was permitted and even necessary for him to kill the Kohanim, nevertheless, according to his level of greatness, Hashem did consider it a sin.]
We need to understand what it was that troubled Moshe. Moshe asked, 'The first King that You anoint over Your children shall be slain with the enemy's sword?'. It seems that he was only disturbed because it was Shaul, who was the first anointed king, that was slain by the enemy's sword. Why would he be less bothered if it was the second or third king, or if he was slain in any other manner other than by the enemy's sword?
Additionally, we need to understand Hashem's response. For Hashem's reply, 'Say to the Kohanim whom he killed, and who accuse him', implies that it was only because of the accusation of the Kohanim whom he killed that Shaul deserved to be slain. Why would he not deserve to be punished for the sin itself, even without their accusation?
The Tribe of Binyamin and the Kingship
The commentaries discuss why it was specifically the tribe of Binyamin who merited to have one of its own tribe members, King Shaul, to be anointed as the very first king over the Jewish People. They explain it as follows. After Yaakov left Lavan's home, he made his way back to his father. On the way, he and his children met his brother Eisav, and in order to appease him they bowed to him in an expression of honor. At that moment when they bowed to Eisav, they had essentially subjugated themselves to him, and no longer had the ability to take away reign over the land from his descendants. Only Binyamin, who was not born at that time and therefore never bowed before Eisav, was able to take their reign away and bring it over to Yaakov's descendants.
Yitzchok's Blessing to Eisav
When Yitzchok blessed Eisav before he passed away (בראשית כ"ז מ) he said, ועל חרבך תחיה - By your sword shall you live. What bothered Moshe was; how can it be that Shaul, who merited to take the reign from the descendants of Eisav due to his being a descendant of Binyamin who never bowed and subjugated himself to Eisav, will be slain by Eisav's descendants over whom he had essentially defeated.
Furthermore, the Zohar (פרשת בלק) tells us that when Hashem offered the Torah to Eisav's descendants they told Him that they, who live by their sword, cannot accept the Torah, which clearly prohibits killing people. Rather, they told Hashem, let Him offer the Torah to the descendants of Yaakov. Hashem asked them, "How can I [so to speak] convince Yaakov's descendants to accept the Torah?" To which they replied, "Bequeath them a portion of our blessing, so that they too can use the sword, at least when punishing those who deserve to die". Accordingly, Moshe was additionally bothered; how can it be that Eisav's descendants, who granted at least partial rights over the sword to the Jewish People, should be able to use the sword to slay Shaul, who was anointed king over the Jewish People?
Hashem's Response
To this Hashem replied, "Say to the Kohanim whom he killed, and who accuse him", with which He was alluding to the following. When Shaul slayed the Kohanim unlawfully, he had essentially used the power of Eisav. By doing so, he had submitted to Eisav's power, in direct contrast to his grandfather Binyamin, who had never submitted to Eisav. As such, in accordance to his own actions, he no longer deserved the kingship which he only inherited due to his grandfather never having deferred to Eisav. And although his grandfather's merit was not diminished, nevertheless because the Kohanim whom he slayed were accusing him, their accusations towards him overpowered his grandfather's merits, and thus Eisav's descendants were able to overcome him. Additionally, because he unlawfully used Eisav's sword to slay the Kohanim, he himself deserved to be slain by Eisav's sword, as well.
('זרע שמשון' פרשתנו אות א)
