So what “threat” are you talking about?
That he’ll do it again when released.
1: Meheichei Teisi he will, and that is a reason to kill a person?!
2: Since when are we “Nidon Al Shem Sofo? The Yodaiah Machashavos said about Yishmael (an Arab!!) B’Asher Hu Sham.
3: Where do you draw the line? Those already in jail? Stone-throwers? Gazans? Arabs?
As mentioned above, Chazal require us to take personal protections against those who are ??? ?????? ?? ????? ????. Never to be pro-active.
Also note: