That is a sign of what has preceded regarding the sending of the messengers to them and warning them of a bad outcome. It is a news of a subject that is omitted: meaning, that matter. And that your Lord was not to destroy the towns is a justification, meaning, the matter we have narrated to you is due to the absence of your Lord destroying the towns unjustly, on the condition that 'that' is what causes actions to be affected. It may also be understood as a lightened form of the heavy one, meaning: because the matter and the discussion was not that your Lord was to destroy the towns unjustly. You may also consider it as a substitute for that, as in His saying: 'And We decreed to him that matter, that the end of these people is cut off, due to the injustice they committed.' Or unjustly, on the basis that if He had destroyed them while they were heedless, they would not have been alerted by a messenger and a book; it would have been unjust, and He is exalted above injustice and all that is vile. And for each of those accountable are degrees based on what they have done, as a reward for their deeds. And your Lord is not unaware of what they do; it is not hidden from Him their measures and conditions and what they deserve in terms of reward.