And why "Venikah ha'Ish me'Avon"? Why might we have thought otherwise?
Rashi and Rashbam #1: To reassure the husband that he will not be punished for bringing his wife to the Kohen and causing her death.
Ramban and Targum Yonasan (on Pasuk 21 DH 've'Zeh Ta'am', citing Sotah, 47b): What the Pasuk means is that the Mei Sotah will only take effect if the husband is himself free of son, by not having been intimate with her from the time that she became a Sotah. 1
Rashbam: The Pasuk is saying that the husband is absolved from the sin of remaining silent in face of his wife's infidelity, seeing as she became forbidden to him. In fact, had he remained silent, and continued living with her, he would have been guilty. 2
Seforno: It means that the Sotah's husband is absolved from the sin of having suspected his wife of having committed a sin which she did not commit. 3
Ramban: Or, as some commentaries explain, if he ever committed an elicit act of Z'nus in his lifetime
And what exactly does the Pasuk mean when it concludes "ve'ha'Ishah ha'hi Tisa es Avonah"?
Rashbam: Refer to 5:31:1:3*.
Seforno: It means that, if she committed adultery, she will die, and if she didn't, she will be humiliated in front of all the people for having had the audacity to contravene her husband's warning.