Stupidity: After Danny kills Kat on the sidewalk, he just stands there with Rory while the other gang members flee to their vehicles. William comes running out of the Mexican restaurant and Rory tells Danny, "We gotta go, brother. Now." Rory sees William and gets into the vehicle without getting Danny to go with him. The gang drives away with Rory watching out the window as William tackles and fights with Danny. Rory and the rest of the violent gang left behind someone who just passed the initiation. (00:07:10 - 00:07:40)
Plot hole: The prosecuting attorney told William "this case [murder of Kat] is so paper thin the judge's probably gonna throw it out" but William could try to convince the D.A. to take it to trial. In addition to circumstantial evidence against Danny and his pending charges (resisting arrest, assault on a police officer, possession with intent to deliver, and parole violations), the random, unprovoked, senseless murder of a teenaged girl would be high profile - such cases are most likely to be prosecuted. (00:09:34 - 00:10:36)