Killing a terrorist when he has stopped murdering

Q: Is it ethical to kill a terrorist when it is logical to assume that he will no longer murder?
A: This question can be divided into two parts: 1. From the perspective of reality, how is it possible to be certain that he has stopped murdering? It is impossible to know. 2. Even if we know that he will no longer murder, we must still kill him. But why – isn't this 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 he 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. The terrorist is not 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 he is later released, 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 with the proper 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 him in order that others will see and be frightened. This "rodef" is teaching other "rodefim" through his action. If he kills Jews and when the police approach, he gives up and we have mercy on him, we encourage others to act like him, thus endangering other Jews. Therefore, in situations like these, we must be extremely ethical. The question is, ethical to whom – the "rodef" or others Jews? Answer: to both of them. We must be ethical to the Jews who have done nothing wrong and to him, since if we kill him, we stop him from killing others and 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 should die liable and not even more liable. We do not use the concept "he should die innocent and not die guilty" to create new laws, but to explain them. C. These are halachot of war, and in war, we do not lock up an enemy who is shooting at us, but 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 pleasant or, in our case, he killed but he will be pleasant. No, we did not make such calculations in a time of war; even a pleasant-seeming non-Jew is killed.
In sum: we therefore see that killing a terrorist is ethical.