Why does the Torah write "v'Im", implying that building a stone Mizbe'ach was voluntary?
Ramban #1, Targum Onkelos and Targum Yonasan: The Torah is saying that, if (when) the time arrives that Yisrael merit inheriting the land and building a stone Mizbe'ach, they should take care that the stones should not be hewn.
Ramban #2 (citing Ibn Ezra): It is said with reference to Mizbe'ach ha'Bris that Moshe built at the foot of Har Sinai. 3
The other two are a. "Im Kesef Talveh" (Shemos 22:24 [See Devarim 115:8]) and "v'Im Takriv Minchas Bikurim" regarding Minchas ha'Omer (Vayikra, 2:14 [See Vayikra 23:9]). Moshav Zekenim - he did not include "Im Kofer Yushas Alav", for it is only if a Mu'ad ox kills a person; it is not always a Chiyuv. Moshav Zekenim (21:30, citing R. Yeshayah)
Why would using a sword on the stones of the Mizbe'ach profane it?
Rashi #1 (citing the Mishnah in Midos, 3:4) and Ramban (citing the Mechilta): Because, since the purpose of the Mizbe'ach is to prolong life, whereas iron (from which a sword is made - Targum Yonasan 1 ) to curtail life, it would not be correct to use the latter in manufacturing the former.
Rashi #2, Ramban #4: The Mizbe'ach creates peace between Yisrael and their Father in Heaven, it would not be correct a sword that causes destruction 2 to manufacture it. 3 (Moreover, Eisav, whom Hashem hates 4 inherited the sword 5 - it portrays his strength both on earth and in Heaven; 6 therefore it is not befitting to use a sword (iron) in the Beis-Hamikdash [Ramban]). 7
Ramban #2 (citing Ibn Ezra): It is to prevent a scenario where the chippings end up in the trash-heap, or where the adherents of Avodah-Zarah take some of the chippings to make a Mizbe'ach for Avodah-Zarah. 8
Ramban #3 (citing Moreh Nevuchim, 3:45) and Rashbam: It is a decree to prevent them from carving pictures on the Mizbe'ach. 9
Ramban (DH v'Zehu ha'Ta'am): However, one may use other metals or the Shamir worm to cut or form the stones of the Mizbe'ach - even though this renders the stones incomplete,
Which is why a sword is called "Cherev" (Ramban).
And if the Torah says this in connection with the stones of the Mizbe'ach, which cannot hear, see or speak, how much more so will no harm befall someone who makes peace between man and wife, family and family and between one man and another (Rashi citing the Mechilta).
In the form of Mazal Ma'adim, the Mazal that depicts bloodshed (Ramban).
Ramban: With the exception of the Shechitah-knives, which were permitted because Shechitah is not an Avodah.
Whose adherents would do so in anticipation of success in their endeavors (Ibn Ezra [bearing in mind the strong Yeitzer-ha'Ra for Avodah-Zarah until Ezra and his Beis-Din negated it]).