I do not think it much of a stretch to say that deliberately causing a person's death (as opposed to alleviating pain in such fashion as may as a side-effect hasten death) is not a legitimate medical function.
The CSA provides that doctors shall be forbidden from writing prescriptions except when they serve a legitimate medical function; it gives the Attorney General some authority to determine whether a particular usage qualifies. I do not think such restrictions are constitutional, but their constitutionality was not raised as an issue in this case.
Stipulating that the CSA is constitutional, I do not see Ashcroft's determination that deliberate death is not a legitimate medical function to be an abuse of power. I find myself very much in agreement with Judge Thomas; IF one accepts that the CSA is wholly constitutional, the legitimacy of Ashcroft's actions must necessarily follow from that. That does not mean that I believe the CSA to be wholly constitutional, but since nobody in this case particularly challenged that, Judge Thomas couldn't either.
They can. There's nothing in the federal law against allowing the people of each state to decide these difficult moral choices. The people of each state have always had the decision.
They just can't use controlled substances to do it. I suspect we'll hear from Congress on this.