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human
[Harry Potter] Polyjuice Potion. I have some questions.

So I was reading up on some history from the second rise of He-Who-Must-Not-Be-Named and came across an interesting find. In the 1994-95 school year at Hogwarts, Barty Crouch Jr. had gone a year impersonating Prof. Mad-Eye Moody, keeping him alive in a trunk. But Why?

Wouldn't have been easier to just have him die and then keep collecting clippings from his corpse? Perhaps you would take the form of the decaying body instead. But what if he were to shave off the hair off Moody early on, then killed him, would the hairs start to decompose and turn the Polyjuice drinker into a corpse or would they retain the life essense of Moody and presented Crouch as a living Moody. 

I just asking just...to know. I'm not planning to drink the potion or anything sketchy like that. A friend wanted to know.
gpt
You already partly answered your own question.

Note that hair and nail clippings are already "dead cells".  They are no longer growing.  Hair and nails both grow from the roots.  

This is actually one of the reasons that the Polyjuice potion has such a short affective (and effective) time span.  

The ingredients of the potion magically enhance the "person ingredient" and apply the transformation/illusion to the imbiber, but in essence "burn off" the "person ingredient" to do so.

Thus, if you were to take clippings from a dead person, it would work a couple times, maybe a few more, but with a living person, you can (somewhat) easily replenish your stock of "fresh person ingredient" to brew new batches of potion.

When the life essence of a person vanishes (i.e. they die) all vestiges of it start to "decay" from their body rapidly.  Eventually, even hair clippings from a dead person will no longer function as a potion ingredient.

Plus, Moody was a powerful enough Auror that if he were to die from anything other than the Killing Curse (which would instantly remove all life essence from his body, hence negating any chance of Polyjuice functioning) he would likely return as a ghost and be able to alert people as to what was going on.