If you want to use the Godfather, you don't seem to recall that the target was left alive, to BE terrorized.
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The movie mogul in the Godfather was left alive so that he would give Johnny Fontaine the coveted part in a movie he was producing. To kill him right away would not have achieved that result. However, if he still refused to give Johnny the part after they whacked the horse, there would be no reason to keep him alive, for any reason, and he would have been killed. The Mafia did not terrorize people because of their religion; it was usually business and the women and children were usually kept out of it. I am not excusing the methods or excesses of organized crime by any means.
But let's get our Godfather straight.
"To kill him right away would not have achieved that result."
Exactly, terrorism's goal is to produce terror and to assume control over the terrorised, the desired result is not simply dead bodies.