Would burning at the stake be a "peaceful" death if the person was jacked up on enough morphine?
What reason is there to regard dehydration as any moreso, aside from the fact that it takes longer?
I am only answering the "how can anyone do something so cruel" part, supercat.
You can't use the pain of dehydration as a reason not to do it if you're the ones not letting the person be sedated.
In my view, it's hypocritical for the state to pretend purposefully starving someone to death is any more moral than giving them an OD of morphine.