Why did he say "Ki b'Sochenu Hashem"?
Malbim: Because [Bnei Yisrael] saw that Hashem dwells among Yisrael and did not hide His 'face' from them, Pinchas knew that the Shevatim [in Ever ha'Yarden] did not sin. There was no doubt about this 1 ; [Yisrael] came to know their reason, and to remove a stumbling block from future generations.
Malbim explained above (verses 16-20), that they thought that the Mizbe'ach is for offering to idolatry or [b'Isur] to Hashem! Was that the Nesi'im's opinion, and Pinchas disagreed? (PF)
What is the meaning of "Az Hitzaltem"?
Malbim: [Had you sinned, you would have] separated 1 (removed) Hashem's Hashgachah ,
Malbim: This is like "va'Yatzel Elokim Es Mikneh Avichem" (Bereishis 31:9).
