Then put him on trial for everyone to see, find him guilty and sentence him to death.
That would be the best result as far as I’m concerned, but he will have counsel, and his counsel will not let this suicidal plea stand. He’ll have him declared insane, and either do a nolo contendre plea, or an insanity defense, if that’s not what this is already, and turn it into a nolo contendre in exchange for life without parole. This won’t see a jury.
He’ll still get to be a martyr and write a rambling manifesto and autobiography from prison, where he’ll be a hero to the Aryan Nation faction. Welcome to the 21st Century.