parshat Pinchas: Why Did Pinchas' Action Stop The Plague?

Perhaps this is obvious. Why did Pinchas' killing of Zimri and Kozbi stop the plague? We read in the previous parsha that as the Hashem became angry when the Israelites committed harlotry with the Moabite women and worshipped Baal Peor (Bemidbar 25:1-3) and this anger was manifest in a plague that killed, until this point, 24,000 people (Bemidbar 25:9). When Pinchas killed Zimri, this plague stopped (Bemidbar 25:8).

Why should more killing - one additional killing by Pinchas - turn away Hashem's wrath?

I would suggest that until this point in this rebellion, no one actively took Hashem's side. Many Israelites were sinning in the two aforementioned ways, and many Israelites did not partake of the sinning. Zimri flagrantly violates Hashem's will, in a public act of rebellion, and in response, Moshe and all the assembled are reduced to tears (Bemidbar 25:6, with an understanding similar to Rashi citing Midrash Tanchuma that וְהֵמָּה בֹכִים פֶּתַח אֹהֶל מוֹעֵד was a reaction to Zimri rather than what was happening at the time).

Thus, it was Hashem vs. many Israelites, and in this scenario, Hashem responded by unleashing a plague on the Israelites for their rebellion. When Pinchas stood up and took Hashem's side, it was no longer Hashem vs. many Israelites, but rather Hashem and some Israelites vs. some other Israelites. It is no longer "Me vs. Them" but rather "Me and some of Them vs. Some other of them." If so, such a harsh response was no longer warranted.

Alternatively, the Israelites are now taking control of the rebellion themselves, and so God does not need to handle it in His wrath.

And so parshat Pinchas begins, in Bemidbar 25:11:

יא פִּינְחָס בֶּן-אֶלְעָזָר בֶּן-אַהֲרֹן הַכֹּהֵן, הֵשִׁיב אֶת-חֲמָתִי מֵעַל בְּנֵי-יִשְׂרָאֵל, בְּקַנְאוֹ אֶת-קִנְאָתִי, בְּתוֹכָם; וְלֹא-כִלִּיתִי אֶת-בְּנֵי-יִשְׂרָאֵל, בְּקִנְאָתִי.	11 'Phinehas, the son of Eleazar, the son of Aaron the priest, hath turned My wrath away from the children of Israel, in that he was very jealous for My sake among them, so that I consumed not the children of Israel in My jealousy.
יב לָכֵן, אֱמֹר: הִנְנִי נֹתֵן לוֹ אֶת-בְּרִיתִי, שָׁלוֹם.	12 Wherefore say: Behold, I give unto him My covenant of peace;

Pinchas turned away Hashem's wrath because he, among them, was jealous for Hashem's sake, such that Hashem did not have to be the only one standing up for Himself, and so Hashem did not consume the children of Israel in his anger.