Why does it say "Im Chai Hashem Yomeru"?
Radak: They would swear in the name of idolatry (and not in Hashem's name) - "va'Yishav'u b'Lo Elokim" (verse 7).
Malbim: If you find that sometimes they swear in My name amidst Mishpat, do not infer that there is Mishpat and Emunah. Just the contrary! They swear falsely, and steal via this. There is no Emunah, for a Shevu'ah means that the matter is as true as Hashem's existence. Since it is false, he denies Hashem's existence 1 !
Even without this, we can say that one who swears falsely shows that he lacks Emunah, for Chachamim taught that there is an awesome punishment for this! (PF)
Why does it say "Lachen la'Sheker Yishav'u"?
Radak: If they do swear in My name, to show that they fear Me, they swear falsely, and [only] I know.
