The Quran

قالَ لَقَد ظَلَمَكَ بِسُؤالِ نَعجَتِكَ إِلىٰ نِعاجِهِ ۖ وَإِنَّ كَثيرًا مِنَ الخُلَطاءِ لَيَبغي بَعضُهُم عَلىٰ بَعضٍ إِلَّا الَّذينَ آمَنوا وَعَمِلُوا الصّالِحاتِ وَقَليلٌ ما هُم ۗ وَظَنَّ داوودُ أَنَّما فَتَنّاهُ فَاستَغفَرَ رَبَّهُ وَخَرَّ راكِعًا وَأَنابَ ۩

Muhammad Asad

Said [David]: "He has certainly wronged thee by demanding that thy ewe be added to his ewes!Thus, behold, do many kinsmen¹ wrong one another - [all] save those who believe [in God] and do righteous deeds: but how few are they!" And [suddenly] David understood that We had tried him:² and so he asked his Sustainer to forgive him his sin, and fell down in prostration, and turned unto Him in repentance.

The term khulata' (sing. khalit) denotes, literally, "people who mix [i.e., are familiar or intimate] with others" or "with one another". In the present instance it evidently alludes to the "brotherhood" between the two mysterious litigants, and is therefore best rendered as "kinsmen".
Sc., "and that he had failed" (in the matter of Bath-Sheba).