I made this tzu shtell years ago, from the Ksav Sofer in pasrshas emor to pshat in pinchas. I think you left out the kicker; The KS explains that the reason kohanim are forbidden from being misasek in kvuras hameis is because it’s the ultimate form of chessed, and kohanim, being descended from Aharon, have a natural predisposition to chessed, and moreso to this ultimate form of chessed. Hashem commanded them to refrain specifically from this mitzvha to learn to exercise control over themselves, this being a middah needed as a leader/Kohen. When pinchas kiled zimri, he exhibited this middah. Though he was a descendant of Aharon, and predisposed to chessed, he overcame that iddah when necessary. And that’s why kehuna was a fitting reward
