Divine justice and inevitable failure of falsehood

فَلَمَّا جَآءَهُم بِٱلْحَقِّ مِنْ عِندِنَا قَالُوا۟ ٱقْتُلُوٓا۟ أَبْنَآءَ ٱلَّذِينَ ءَامَنُوا۟ مَعَهُۥ وَٱسْتَحْيُوا۟ نِسَآءَهُمْ ۚ وَمَا كَيْدُ ٱلْكَـٰفِرِينَ إِلَّا فِى ضَلَـٰلٍ

And when he brought them the truth from Us, they said, "Kill the sons of those who have believed with him and keep their women alive." But the plan of the disbelievers is not except in error.