People—lawyers or not—who threaten to start criminal proceedings against others can land in hot water.
Lawyers. Lawyers who threaten to take opponents to criminal court in order to gain an advantage can be subject to discipline for unethical behavior. This scenario sometimes arises in particularly contentious cases: Lawyer A threatens to report Lawyer B or Lawyer B’s client to the authorities for some kind of alleged misconduct unless they cave to A’s demands. Of course, if the threatened party has actually committed a crime, that party probably won’t want to report the threat-maker (thereby potentially drawing the interest of law enforcement investigators). Whether a lawyer who makes this kind of threat can be disciplined depends on the law in the jurisdiction. In California, for one, the relevant rule says that a lawyer “shall not threaten to present criminal, administrative, or disciplinary charges to obtain an advantage in a civil dispute.” (California Rule of Professional Conduct 5-100(A) (2017).)
Members of the public. It can even be a crime to threaten to initiate criminal proceedings against someone. In Washington, for example, trying to obtain goods or services from another by threat of criminal prosecution constitutes second degree extortion. (Wash. Rev. Code § § 9A.56.110, 9A.56.130, 9A.04.110 (2017).) (It’s a defense that the defendant reasonably believed the threatened charges to be true and was acting solely to cause the other party to remedy the situation.)
Debt collectors. It can also be illegal for a debt collector to threaten criminal action against someone who is late on a bill. (See this article on debt-collector threats.)
Anyone who’s been threatened with criminal charges should normally consult an experienced attorney as soon as possible.