Your question goes to the heart of the difficulty in making ethical decisions, which is when you are weighing two positive values against each other. The question does really suppose two questions: What are the limitations of Pidyon Shvuyim, namely is there a price that is too high to pay to redeem captives? Second, what is the conception of justice particular in reference to criminal justice?
With respect to the first question, the mishna in tractate Gittin (4:6) that we do not redeem captives for more than their value because of Tikkun Olam. The Gemara there posits two reasons for this. One that it would pose an extreme difficulty on the community that would need to pay the ransom and the second reason is that it would provide an incentive for kidnappers to take more Jewish captives in the future. Later commentaries and halachic sources understand the value here to be the value on the slave market.
The concern that if the Jewish community pays too much for captives other Jews would be taken captive has been a real one. There is in fact the famous case of Rabbi Meir of Rothenberg, known popularly as the Maharam of Rothenberg, who was imprisoned in 1286 by King Rudolph I and demanded a large ransom. The Maharam’s student, Rabbeinu Asher ben Yechiel, known as the Rosh attempted to raise the ransom, but the Maharam refused it, lest it provide incentives for other Rabbis to be imprisoned. The Maharam died in prison seven years later in 1293.
The notion of not paying too much for captives does come with exceptions in later halachic sources. For example, the Shulchan Aruch (Yoreh Deah 252:4) rules that one paying his own ransom may pay more than his worth. Earlier authorities debate whether one may also pay more to redeem his wife. The Shulchan Aruch also rules that the community may pay more for a Torah sage.
More relevant to the case of Sgt. Gilad Shalit, many later authorities understand the limit on not paying more than the value of captives only applies when the motive for their capture was financial gain on collecting ransom. However, if there is a real risk that the captive will be killed, then one may pay an unlimited amount (Aruch Hashulchan YD 252:11).
It would therefore seem from the perspective of these sources on Pidyon Shvuyim, as Sgt. Shalit’s life was likely endanger, a high price for his release is warranted. However, his case does bring the concern that the Gemara had that there would be an incentive to capture more Israeli soldiers. Furthermore, there is the concern of your question that many prisoners to be released have engaged in terror and murder and might likely do so again in the future.
This dilemma has been a source of debate among prominent Israeli Rabbis for decades as well. The former Ashkenazi Chief Rabbi Shlomo Goren believed that Israel should not release prisoners for soldiers on the grounds it would put the lives of other soldiers at risk. The prominent Sephardi Authority Hayim David Halevy, on the other hand, was a vocal proponent of such exchanges.
An additional factor in these decisions which I will not discuss in detail is the compact an army makes with its soldiers – namely the notion of “leave no man behind.” The army tells its soldiers, if you fight for us and your country, your country will do everything to make sure you come home, regardless whether that is dead or alive. Leaving a soldier in captivity might send a message to other soldiers that your country could abandon you if the price is too high.
With respect to the second question about the Jewish conception of justice, we need to first understand what is desired in imprisoning criminals. In the general world, imprisoning prisoners is done for several reasons – protecting society from people who are a danger to society, punishing the criminal, and serving as a deterrent to others from committing the same crime.
Not of all of these reasons are values equally in Judaism. Judaism is very concerned with protecting society from those who are dangerous. Our conception of justice also seeks for a criminal to make retribution for his/her crimes, but there the motive is so that the criminal can achieve some atonement from God and not from a pure desire to punish the criminal.
I think the distinction can be understood through two different Mishnayot. The first found in Makkot (1:10) states that a Sanhedrim that kills one person in a week is called destructive. Rabbi Elazar ben Azaryah says, once in seventy years (would be destructive). Rabbi Akiva and Rabbi Tarfon say that if they had been on the Sanhedrin, they would never kill anyone ever. Rabbi Shimon ben Gamliel says that they would be increasing murderers in Israel.
The second mishnah in Sanhedrin 9:5 states that one who has received lashes twice, the Beit Din would put him in a cell and feed him barley until his stomach explodes. One who murders without witnesses (or two kosher witnesses) is placed in a cell and fed with bread of adversity and water of affliction. The Gemara later says that this murderer is also fed the barley until his stomach explodes.
The mishna in Makkot (with the exception of Rabbi Shimon ben Gamliel) seems very concerned with punishing known criminals by putting them to death. The Mishna in Sanhedrin, however, seems to have no qualms with putting to death people even without the proper evidence necessary for Beit Din and hence the execution is done passively through the prison diet.
I think the two Mishnas can be explained by saying that the Rabbis were very reluctant to say that a person deserved to be “punished” or even lose his life according to the Torah and
G-d’s law. At the same time, the Rabbis felt the need to protect society from dangerous people even if it meant their execution.
With respect to the prisoners released in exchange for Sgt. Shalit, is our concern that they would still pose a danger to society or does it simply just bother us that they would be “getting away with murder.” The latter might be a valid concern, but one the Rabbis would be more reluctant to employ when weighed against the life of a Jewish captive. The former is more serious and one to the Rabbis would be very concerned with.
The Israeli government clearly needed to weigh the risks and dangers of the future actions of each released prisoner and there is much to debate there.
This discussion seems that this was never going to be an easy decision. Jewish tradition and law can indicate to us the pros and cons on each side of the debate, but is unlikely to settle the issue for us, and we are likely to debate such exchanges again in the future.
However, at this point, Sgt. Shalit is now home with his family and we should all rejoice in that fact and hope we don’t need to debate this in the future.