Complex sentence: As early as April, Đujić's Chetnik's began killing and mutilating Croats in retaliation for the wave of Ustaše atrocities that had befallen Serb communities across the NDH around the same time.
Split into simpler sentences: As early as April, Đujić's Chetnik's began killing and mutilating Croats. This was done in retaliation for the wave of Ustaše atrocities that had befallen Serb communities across the NDH around the same time.