My understanding of manslaughter is that it applies to events when someone dies as a result of other unlawful acts and the death is not intentional. Given the information apparently available to the jury in this case itappears to be appropriate. My understanding is that the sentence is servere. Under the circumstances it would appear that justice has been served.
What concerns me, however, is what are we doing as a society, culture, that permits the development of such failures of humanity as these miscreants? They are barely adult and by the time they have completed their sentences thay will be middle-aged.