I'm pretty sure there's no plea bargain he can get other than protection for his family.
The most common arrangement is "plead guilty to spare us years of bullshit and we'll take the death penalty off the table," with the idea being you spare the survivors the discomfort of testifying and/or giving victim impact statements, saving the state tons of money and time prosecuting the case, and reducing the availability of appeals (most plea deals include a waiver of the right to appeal, which doesn't cover
everything but substantially reduces the potential burden on the state), and then the unspoken delight of depriving a dirtbag defense attorney getting a free pay day defending a hopeless cause.
The downsides, of course, are myriad -- no death penalty, no slam-dunk guilty verdict from a jury for the prosecutors (i.e. not as high-profile, so not a big career booster), victims who
want to give impact statements lose that opportunity, no media spectacle, and as you argued for this case there's definitely a benefit to dragging this shithead through the mud in court just to show the public how fucked up the ideology has gotten.
The courts prefer plea bargains, of course, because it makes the job easier, and they have their ways of discouraging full trials. Whether the feisty DA and governor along with general public outrage compels a full trial remains to be seen.
