وَإِنْ شَاءَ عَذَّبَهُمْ فِي النَّارِ بِعَدْلِهِ
And if He wills, He punishes them in the Fire according to His justice.
Justice means punishment for sin is not arbitrary oppression. This preserves the seriousness of major wrongdoing. Forgiveness by grace and punishment by justice are both possible; neither outcome may be imposed as a certain fate for every sinful believer.