Torah law is used as long as society functions properly/Igros Moshe

Igros Moshe(C.M. 2:68): … The Torah reserves capital punishment for those sins which are very serious such as murder, kidnapping, sexually prohibited relations and idolatry. The perpetrator in these cases is unrestrained and is capable of doing whatever disgusting and cruel acts in the world that are in his heart that he thinks are for his benefit. However the death penalty is not administered out of hatred to evildoers or fear for the welfare of society because Bava Metzia (83b) tells us that G‑d will punish transgressors. That in fact is the halacha as poskened by the Rambam (Hilchos Chovel u’Mazik 8:9) as well as all other poskim. So on the one hand the purpose of capital punishment is to let people know the severity of these prohibitions so that they will not transgress them. On the other hand the laws of capital punishment emphasize the importance of each soul and other concerns. Therefore we are commanded that only the Sanhedrin with proper semicha can judge these cases. Only the greatest people in Torah scholarship and other knowledge receives this semicha. In addition to their knowledge they also need to have perfected their character and be very humble as well as G‑d fearing people. They also need to hate money and love the truth as well as wonderful people who are beloved by all…They don’t have any imperfections or bad reputations and they are very merciful. That is why very old people are not appointed judges because they have forgotten the stress of raising children. Also people without children are not appointed because they lack mercy to some degree and they will be too angry at those who have committed sins. Even these great and good people cannot judge unless they constitute a Sanhedrin of 23 people. However it is not enough there are 23 such people to make a Sanhedrin. They also need to have before them 3 rows of very great Torah scholars who are not yet great enough to be part of the Sanhedrin – but are almost great enough. This is to protect the Sanhedrin from making a mistake in judgment. That is because when these three rows of scholars think that the Sanhedrin is mistaken in their ruling of innocence they will protest and will not listen at all to their words. Another safeguard against making a mistake is that they do not convict based on circumstantial evidence – no matter how convincing. They only convict a person based on two valid witnesses who have not the slightest bias in the matter... Furthermore the witnesses are warned concerning the severity of the sin of false testimony as well as the seriousness of the sin of murder so that they are very afraid of mistakenly convicting or mistakenly declaring the suspected murderer innocent. Even with all of this the witnesses also have to warn the person against murder and the suspected murderer has to acknowledge the warning by saying that even though he is aware of the seriousness off the crime he is still doing it. As a consequence of all these safeguards, only once in many years would someone be convicted of murder. In addition it was impossible to judge capital cases unless the Temple existed and that the Sanhedrin of 71 of the greatest scholars was in session on the Temple Mount. In fact capital cases were not judged even in those countries where the king gave the Jews permission to judge their own people according to the law of the Torah. As a consequence of these two factors there were almost no Jewish murderers because of the awareness of the severity of the prohibition of murder and because they were educated by means of the Torah and the punishments of the Torah to understand the seriousness of the crime. They were not simply afraid of punishment in the sense of getting caught but were afraid of the crime itself. However this use of the Torah system to run society was only when the crime of murder was not common but was simply the result of someone’s great lust or some quarrel concerning money or honor. But when people killed simply because it was viewed as an insignificant thing and the murderer was simply a callous and cruel person or similarly if there was a great deal of murders and wickedness – then a different system of law was utilized that was concerned with the pragmatic question of stopping killing and the goal became saving the society