By definition, if it's insulated he couldn't have received a shock. So there must have been a break in the insulation or the insulation was not sufficient to block the voltage that allowed the current which electrocuted him
What if the wire was intended for a low frequency but got shorted to a high-frequency, high-voltage supply? It might be possible for the conductor-insulator-hand to form a capacitor permitting enough AC current to flow to be fatal.