Question: Is it ethical to kill a terrorist when it is logical to assume that he will no longer murder?
Answer: This question can be divided into two parts: 1. Can we really be certain that he has stopped murdering? It is impossible to know. 2. Isn't this similar to the law of a "Rodef" (literally "pursuer" - a case in which one is permitted to kill a pursuer so that the pursued person is saved from harm)? If the "Rodef" is in pursuit, we kill him, and if he is not in pursuit, we do not kill him.
There are three answers given by halachic authorities:
a. A terrorist is never finished being a "Rodef". He is not an "individual Rodef" who is angry with a particular person and wants to kill him. He is a "communal Rodef" who wants to kill Jews and he does not care which Jews they are. If we capture him, put him in jail, and later release him, as is the custom – to our great distress – he will continue to murder. The organization of parents of those murdered by terrorists has exact records which state that more than 180 Jews have been murdered by released terrorists who have murdered again. This means that when you free a terrorist - even with the goal of helping Jews - you endanger more Jews. This person is therefore not a one-time "Rodef," but a perpetual "Rodef."
b. The halachic authorities also say that you should kill the terrorist in order that others will see and be frightened. This "Rodef" is teaching other "Rodefim" through his actions. If we have mercy on one who kills Jews and then gives up when the police approach, we encourage others to act like him, thus endangering other Jews. In situations like these, we must indeed be extremely ethical. The question is, ethical to whom – to the "Rodef" or to other Jews? Answer: to both of them. By killing him we are ethical to the Jews who have done nothing wrong, and we are being ethical to him, since we stop him from killing others and thus lessen his "Gehinom" (punishment in the World to Come). The Mishnah in Sanhedrin (71b) says that the "Ben Sorer U-Moreh" (the rebellious son – see Devarim 21:18-21) is killed on account of his future. While he has done many things wrong, he has not committed a sin for which he is liable for capital punishment, but he is killed so that he will die innocent and not guilty. In our case the terrorist is already liable, but he will die less liable than if we let him live. We do not use the concept "he should die innocent and not die guilty" to create new laws, but to explain them.
c. There are halachot of war. In war, we do not lock up an enemy who is shooting at us: we fire back at him. This is similar to what King Shaul said to the "Keni" (Shmuel 1 15:6): "Go, depart, go down from among Amalek, lest I destroy you with them." This means, even though you are my friend, if you are there, you could get hurt or killed. In the halachot of war, we do not make such calculations, as it says, "The best of the non-Jews should be killed." The Tosafot raised a major difficulty with this statement: how can we say such a thing when according to halachah it is forbidden to kill a non-Jew and all the more so the best of the non-Jews (Tosafot to Avodah Zarah 26b and see Beit Yosef Yoreh Deah 158)? Tosafot explained that this statement refers to a time of war. This non-Jew seems harmless or, in our case, he killed but he will become harmless. No, we did not make such calculations in a time of war, even a harmless-seeming non-Jew is killed.
In sum: We therefore see that killing a terrorist is ethical. However, this matter must be decided by the Chief of Staff and the officers of Tzahal and not by individual soldiers. The State of Israel has signed international agreements which regulate warfare, and we must abide by them. Although in specific instances we might suffer from these agreements, overall they are beneficial to us.