Why wouldn’t the shooting be justified if the thief still had the wire he was stealing in his hands.
In most jurisdictions, it is not lawful to use deadly force to protect against minor property loss. There are exceptions, but that is the way it is in most places. The value of the property stolen might not have even made it to the level of a felony.
You are not allowed to use deadly force to stop minor crimes if no one’s person or life is in danger.