In ayah 79, Allah subhanahu wa ta’ala explains the reason behind such an intense punishment [mentioned in ayah 78]. He says,
كَانُواْ لاَ يَتَنَـهَوْنَ عَن مُّنكَرٍ فَعَلُوهُ لَبِئْسَ مَا كَانُواْ يَفْعَلُونَ
They used not to forbid one another from the evil they committed. Vile indeed was what they used to do.
They did not stop each other from committing sins and prohibitions. If a person is doing an evil act himself, would he stop others? No, he will not. It is the duty of the scholars and the educated ones to stop others from doing evil acts, but when they themselves are involved in wrong acts then who will stop others?
– When a person sees something wrong and he doesn’t stop then this brings about the curse of Allah subhanahu wa ta’ala.
= When a person has emaan he becomes too sensitive and firm about the permissible things. He will neither do any wrong act himself nor allow others. He will speak up against injustice and transgression. If a person sees something evil and doesn’t stop then this shows there is a problem with his emaan.