But it’s also important to bear in mind the context of the “witness” identification (if indeed there was one). No one ever witnessed JtR actually killing his victims. The nature of the witness who saw the murderer (if indeed this happened altogether) was that he saw a guy in the right time and place, such that the police believed that that guy must be the same person who committed the murder. But it’s very possible that the witness himself was considerably less certain of this, and that this uncertainty played a big role in his reluctance to testify. It’s not at all comparable to a case where someone sees a someone commit a murder but refuses to testify against him.
There is a principle in Judaism that “the law of the land is law” and that it is a Torah obligation to follow the law of the land, but that only applies when said law does not go against the Torah’s own laws. (For example, if infant circumcision were to be outlawed by the state, the religious rules would say to circumcise on the eighth day and to defy the laws of the land.) The laws about when one Jew can subject another Jew to a non-Jewish legal proceeding are not simple, and given the attitude of the huge majority of non-Jewish rulers toward the Jews under their dominion, it should not be surprising that the attitude of “never give up a fellow Jew” has tended to be the default assumption. That said, under proper circumstances…including (but not limited to) when the Jew being handed over is a predator whose continued freedom from consequence is a danger to the community at large…all prominent Rabbis have stated that it is not only permitted, but required to help the authorities to apprehend him or her.
One of the first rules of Jewish law is that, if it would save a life, the law must be broken. I work in a store owned by Orthodox Jews. At the first sign of any disturbance, we have to call police. Why? Because we don’t want it to escalate to the point where someone gets killed.