You bring up an interesting case – my take on that would be that if a criminal is caught and subject to the penalty of the law, then so far as the law is just, they are correctly subject to the punishment as the government has a just claim on them which should be proportionate to the crime. If, however, the government can see a means to bring a greater degree of justice by offering clemency to the first criminal in exchange for cooperation in capturing other criminals who represent a greater quantitative or qualitative ‘evil’ then that is a form of exchange which is not inconsistent with the idea of rational self interest.
It does put some mixed incentives into play which murkey the water a bit – I have heard of DA’s proposing much more serious sentences as a threat to the criminal in an effort to coerce them into cooperation. To the extent that there is an element of coercion involved, it cannot be considered a voluntary action. As the criminal is already subject to justice based on their prior actions, any negotiation that they engage in is not your standard voluntary fare.