I would think that the person who did the beating, would be an accessory. Those who planned and carried out the dehydration would still be guilty of murder, with or without a determination regarding the beating.
We will have to let the docs have a few mysterious remarks on this one :-) Dr. Thogmartin gave the MECHANISM of death as dehydration (adding that it was not starvation). For the CAUSE and MANNER of death, he hearkened back to Terri's "severe anoxic brain injury," adding that the cause thereof could not be determined with any degree of medical certainty. In other words, just as we knew all along -- her brain was damaged by not getting oxygen, cause unknown. If you prefer, in another passage, he called it "marked global anoxic-ischemic encephalopathy resulting in massive cerebral atrophy."
Same translation. Something or someone cut off her oxygen and we know no natural cause for it. I occasionally, just occasionally, can imagine a not-so-natural cause for it. Blonde hair, for instance :-)