I know of no Ohio law directly addressing the question. I suspect, in the real world, the warden would order a temporary halt to the execution and call the Director of Corrections or even the Governor for orders. The best practice would, I think, be for the execution to be stayed in order for evidence of the guard’s murder to be gathered. The family of the guard might or might not want a new trial for the guard’s death, but the prosecutor of the county in which the prison was located would have the discretion not to seek a grand jury indictment of the prisoner for the guard’s murder. TPTB would probably decide just to let the original execution go forward, especially if “everyone knows he did it.” The defendant can only die once, after all.