The box was found sitting on the sidewalk a few blocks away, with the heads still in it.
One can only imagine what went through the thief's mind when he opened his "prize".
Body parts are only super custody tracked when they are still viable lifesaving tools (ie organ transplant). Besides that they aren't a huge deal.
Reading up on his background, I wonder if he just snapped and killed the guy out of anger for whatever reason and then started cannibalizing the corpse so that he could play the mental illness card at court.
It is likely this is largely true, even if not formally banned from driving, so the answer is probably yes to your hypothetical they should go to prison, or pulling the normative phrase out, likely would go to prison (subject to many mitigating factors).
Even if we accept your premise that having a mental illness precludes one from taking responsibility for their actions, you still have to deal with the fact that they are a serious danger to society. One of the uses of prison is to take people who would otherwise murder and eat people and remove them from society, so that they can't do that.