As I read the article he admits to running the man over, he and his mom state that they pulled him from the windshield, felt a pulse, left him on the ground and drove off.  The driver admits to covering his car with a tarp.  His mother testified to those things in front of the Grand Jury.  I don't know the threshold for Second Degree Murder is, but the Grand Jury was given the law in detail and decided for a true bill.  Zephyr is right that they showed an absolute void of any human decency.