I believe that attempting to file charges would indeed be unethical in most states as well as pointless. Most states base their rules of professional conduct on the American Bar Association (ABA) model rules of professional conduct. Rule 3.1 states:
A lawyer shall not bring or defend a proceeding, or assert or controvert an issue therein, unless there is a basis in law and fact for doing so that is not frivolous, which includes a good faith argument for an extension, modification or reversal of existing law. A lawyer for the defendant in a criminal proceeding, or the respondent in a proceeding that could result in incarceration, may nevertheless so defend the proceeding as to require that every element of the case be established.
Knowingly filing charges against a dead suspect would be frivolous and a waste of the court’s time (and that of the prosecutor) because as Mr. Knowitall points out, it is impossible to convict (or even arraign or put on trial) a dead defendant. The rights guaranteed a defendant in the Constitution alone make that legally impossible, and the practical problems also make it impossible as well. Moreover, Model Rule 3.8 imposes special ethical responsibilities on prosecutors that again would be impossible to meet with a dead defendant. All this taken together, I think it would violate the prosecutor’s ethical obligations to do this. How severely the state’s disciplinary body would sanction a prosecutor for it is another matter. It might not draw a very severe consequence. The particular facts would matter.