وَإِذْ قَتَلْتُمْ نَفْسًا فَادَّارَأْتُمْ فِيهَا ۖ وَاللَّهُ مُخْرِجٌ مَّا كُنتُمْ تَكْتُمُونَ
Wa iz qataltum nafsan faddaara'tum feehaa wallaahu mukhrijum maa kuntum taktumoon
And remember when you killed someone and quarreled about it — but God was going to bring out what you were concealing.
Iddāra'tum is a reciprocal form of a verb meaning to ward off or push away, so what the parties are doing is pushing the matter at each other. Arabic builds the mutual sense into the verb itself, which English needs a phrase to carry. The killing is mentioned here for the first time, after five verses about the cow, so the passage gives the demand before it gives the reason.