Thoughts for now..
Total speculation of course, but one way i can see the attack going down is if KJ had a leash that can release more line, ie.KJ is walking close to the path, extends the leash allowing the dog to enter the park to do business, or not, while KJ stands a bit in the distance, perhaps distracted with a phone, or not.
Perp not immediately seeing dog, suddenly lunges/slashes at KJ and the dog comes running at perp who lifts whatever weapon he has in hand and fatally kills or 'neutralizes' dog.
KJ is pulled into park.
The weapon may be a knife, maybe a long one like a sword, maybe even a machete or who knows what, but thinking if it was long it would be able to reach the dog before the dog reached the perp.
speculation, imo, fwiw.