וַיֹּ֙אמֶר֙ לְכָ֣ה אִתִּ֔י וּרְאֵ֖ה בְּקִנְאָתִ֣י לַיהוָ֑ה וַיַּרְכִּ֥בוּ אֹת֖וֹ בְּרִכְבּֽוֹ׃
가로되 나와 함께 가서 여호와를 위한 나의 열심을 보라 하고 이에 자기 병거에 태우고
Rashi
In his zeal to avenge the Bnei Yisroel. Because he [Shaul] intended to clean and purify Yisroel and to take care of their [Yisroel's] needs, he wanted to kill them. This anger was for a good purpose as we find "Are you zealous for my sake?"9Bamidbar 11:29. And we also find "by the zeal for God's sake" as was said of Yeihuh.10See Melochim I, 19:10. This is the simple explanation of our verse. [But] according to the Midrashic intepretation, he [Shaul] only ordered that the kohanim be killed. And the zeal described here was not for a good purpose but rather for a bad purpose because he [Shaul] knew that his kingdom would not continue further from the day that he did not heed God's commandment concerning Amoleik,11Shmuel I, 15:9. and from that time on, he was angry [and persecuted] them.