Albert Barnes Commentary Ezra 6:12

"and the God that hath caused his name to dwell there overthrow all kings and peoples that shall put forth their hand to alter [the same], to destroy this house of God which is at Jerusalem. I Darius have made a decree; let it be done with all diligence." — Ezra 6:12 (ASV)

Destroy all - A similar malediction is found at the end of the great inscription of this same king Darius at Behistun: If anyone injures the tablet which he has set up, he prays that Ormazd will be their enemy, and that they may have no offspring, and that whatever they do, Ormazd may curse it for them.

To alter and to destroy this house - that is, to alter the decree, and then proceed to destroy the house.