John Gill Commentary Nehemiah 4:5

"and cover not their iniquity, and let not their sin be blotted out from before thee; for they have provoked [thee] to anger before the builders." — Nehemiah 4:5 (ASV)

And cover not their iniquity, and let not their sin be blotted
out from before thee
Let it not go unpunished, and even let it not be pardoned; which is spoken, not from a private spirit of revenge, but from a public spirit for the glory of God, and his justice; and not as a mere imprecation, but as a prophecy of what would be the case, in like manner as many of David's petitions in the Psalms; and for this there was a good foundation, since God had threatened the Moabites and Ammonites with utter destruction:

for they have provoked thee to anger before the builders ;
by despising his people, and mocking at the work the Lord had called them to; and this they did publicly, and on purpose to discourage the workmen.