ذَٰلِكَ بِمَا قَدَّمَتْ أَيْدِيكُمْ وَأَنَّ ٱللَّهَ لَيْسَ بِظَلَّٰمٍۢ لِّلْعَبِيدِ
That is for what your hands have put forth [of evil] and because Allah is not ever unjust to His servants."
(This is for that which your own hands have sent before (to the Judgement)) this is what your hands have done while you were in a state of idolatry (and (know) that Allah is not a tyrant to His slaves) such that He would take them to task without them having committed a crime.