Мидраш к Бамидбар 25:11

(Numb. 25:11) “Phinehas ben Elazar […].” What reason did the Holy One, blessed be He, have for tracing the lineage of Phinehas after this act (of slaying Zimri in Numb. 25:8)?¹Numb. R. 21:3. [The reason was] that, when Zimri was pierced along with Cozbi, the tribes rose up against [Phinehas] and said, “Did you see the son (actually, grandson) of Puti? This man, whose mother's father²Thus Puti was actually Phinehas’ maternal grandfather, whose full name was Putiel. So Exod. 6:25. fattened calves for idolatry, has killed a tribal prince of Israel!” Therefore Scripture has come to trace his lineage [through his paternal grandfather] (in Numb. 25:11), “Phinehas the son of Eliezer, [who is] the son of Aaron the priest.” (Numb. 25:12:) “Therefore I hereby grant My covenant of peace.” Great is the peace that He gave, as the world only functions according to peace. And the Torah is [likewise] completely peace, as stated (Prov. 3:17), “Her ways are pleasant ways, and all her paths are peace.” If someone comes from a journey, we inquire of his peace (wellbeing). So too in the morning, we inquire of his peace, and in the evening we inquire of his peace. And we read the recital of the Shema and we conclude [its blessings] with peace, “who spreads the cover of peace.” And in prayer, we conclude, “Who blesses His people Israel with peace.” (Numb. 25:12:) “Therefore I hereby grant [My covenant of peace],” as he still survives. And so it says (in Mal. 2:5), “My covenant was with him, [a covenant of] life and peace.” (Numb. 25:13:) “And it shall belong to him and to his seed after him […, because he was zealous for his God] and atoned for the Children of Israel.” Since atonement is spoken of in connection with him, did he offer a sacrifice? [The mention of atonement is] simply to teach you that when anyone sheds the blood of the wicked, it is as if he had offered a sacrifice.

Another explanation. They sinned at Shittim, they were smitten at Shittim, and they were healed by means of Shittim. They sinned at Shittim; as it is said: And Israel abode in Shittim (Num. 25:4); they were smitten at Shittim, as it is said: And those that died by the plague (ibid., v. 9); and they were healed by means of Shittim, as it is said: Boards of acacia (shittim) wood. You find that they did not leave that place until Phinehas arose and turned away His wrath, as it is said: Phinehas the son of Eleazar, the son of Aaron the priest, hath turned My wrath away from the children of Israel (ibid., v. 11). The Holy One, blessed be He, declared: In the world-to-come I will heal at Shittim, as it is said: And it shall come to pass in that day, that the mountains shall drop down sweet wine, and the hills shall flow with milk, and all the brooks of Judah shall flow with waters, and a fountain shall come forth of the house of the Lord, and shall water the valley of Shittim (Joel 4:18).

"Assail the Midianites" - Why? "For they assailed you" (Numbers 25:18). From here the sages said, 'rise and kill the one who comes to kill you.