Tafsir of As-Saffat 37:31

This is a corollary drawn from the explicit meaning of what preceded—regarding the lack of faith among those who disputed with them and their nature as people who were transgressive in their own essence—and from what their disputation necessitated and implied regarding the disbelief of those who responded to these transgressors, as well as their own inherent misguidance.

The plural pronouns refer to both factions. It is as if they said: "And because we all, in our very nature, were not believers and were a transgressive people, the word of our Lord—our Creator, who knows what we are and what our predispositions necessitate—has become binding upon us. His threat, may He be glorified, has been established against us: that we are inevitably tasters of His torment, Almighty and Majestic is He."

Their intent is that the true source of the dispute—which is the torment—is a decreed matter from which there is no escape. It has been ordained for each of us due to a state that is within ourselves, necessitated by our own predispositions and actions chosen by our own free will. Therefore, let not some of us blame others; rather, let each of us blame himself. They included themselves with them in this statement to emphasize the closing of the door to blame and dispute from those people.