it remains hard to imagine a jury that would convict, fully knowing he actually killed the mass murderer. reasonable doubt might almost work againt mangione

Yup. All it will take is one juror who understands Jury Nullification [0,1], and they've got at best a hung jury mistrial...

[0] https://www.law.cornell.edu/wex/jury_nullification

[1] https://en.wikipedia.org/wiki/Jury_nullification