c. But the jury’s conclusion that Hinckley was insane is now binding on the government, and thus precludes a retrial for murder: Under the “collateral estoppel” doctrine (Ashe v. Swenson (1970)), “when an issue of ultimate fact has once been determined by a valid and final judgment, that issue cannot again be litigated between the same parties in any future lawsuit.” That’s most commonly applied in civil cases, but it also applies in criminal cases, against the government (so long as it’s the same government involved in both cases).
The jury determined by a valid and final judgment that Hinckley was insane, and thus couldn’t be liable for attempted murder. This judgment is binding on the government, and since the insanity defense applies the same way for murder as for attempted murder, it means that Hinckley would now be conclusively presumed to have been insane for purposes of any murder prosecution as well. He would have an ironclad defense to the murder charge, and thus any case against him couldn’t proceed. For a similar case, see United States v. Oppenheimer (1916), though there the defense was the statute of limitations rather than insanity.