Parshat Balak concludes with the story of Pinchas killing Zimri, the prince of the tribe of Shimon, for behaving promiscuously with a Moabite. In Parshat Pinchas, Hashem rewards Pinchas for his zealousness by giving him a “covenant of peace.” Hashem usually punishes and rewards Midah KeNegged Midah, a response befitting the action. Why would Hashem reward someone who has just killed another man by giving him a covenant of peace? Furthermore, Pinchas was the grandson of Aharon, who was known as an “Ohev Shalom VeRodeph Shalom,” one who loves and pursues peace. One would expect that an individual raised in such a peaceful atmosphere would himself be peaceful and not involved with acts of murder and revenge.
The Ibn Ezra posits that the promise of peace Pinchas received was necessary to protect him from the other members of the tribe of Shimon. Having just killed the leader of the tribe, Pinchas might have been attacked by the members of the tribe in reprisal. Hashem therefore assured Pinchas of his safety.
Rav Aharon Lichtenstein offers a second answer. He quotes a Gemara in Sanhedrin (82a), which derives from “And he got up from amongst the congregation and he took a spear in hand” (25:7) that it is forbidden to enter a Beit Midrash with a weapon. Because Pinchas had to get up to take a weapon, the Gemara assumes that he generally stayed in the Beit Midrash. He rose from his normal conduct and spirit in order to correct the immoral wrong that had been committed by Zimri. This teaches us another thing about Pinchas. He had the mettle to stand up do what was required while the rest of the nation was in a state of crisis. Thousands of Bnei Yisrael had participated in the idol worship of Peor, plague was rampant among the nation, and the leadership had broken down. Pinchas realized that his radical act, a demonstration so contrary to his innermost beliefs, was necessary to right the wrongs and lead Bnei Yisrael back to a state of calm order. It is now understandable why Pinchas needed a promise of peace. He has just broken a psychological barrier within himself by publicly killing another Jew. Regardless of the years he had spent living a tranquil life studying, there was the danger that he had lost the sensitivity towards human life as soon as he killed Zimri (see the comments of the Netziv). It was therefore imperative that he receive a covenant of peace, an assurance that he would be able to return to his natural and desired path.