We really don't know much about the actual crime scene, apart from what has been released.
I am going to guess, if it was an acquaintance, he/she could easily have greeted KJ, and bent down to greet Bowie. At that time, slip a garrote over the dog's head, and choke the dog. KJ would have been in shock, as anyone would be, then out comes the knife. Done.
Wearing black, takes care of the blood at night. Walk away. Wearing black shoes, black stretch pants, black shirt, maybe put on a black jacket stashed. Put on a black watch cap. Look like any other joggers.