"The killing of Dykes was justified because: he was actively and certainly and verifiably engaged in threatening the life of the child. "
And Americans actively supporting terrorists abroad are engaged in treats to Americans everywhere. There is no difference. Dykes could have surrendered to face charges, and the Americans on a "hit" list can likewise surrender and face charges. There is no difference.
"No one claimed it was justified because of any prescience of his future actions,"
You just did yourself -- a threat is a prediction or a concern about future actions.