I have this Idea for an end-quest Boss for the mid-campaign. He's a druid hell-bent on letting nature reclaim everything, favoring the destructive and predatory side of the wilderness. I would like suggestions on how to build this level 13-20 Moon Druid to be as destructive as possible, and how to roleplay him as an insane shapeshifting serial-killer. I think this would be an interesting guy to fight and roleplay as. Happy Halloween!

It sounds like your villain may be a better fit for the Circle of Wildfire as destruction is part of their M.O., however, continuing with the shapeshifting theme:

The Druid Capstone ability allows you to Wildshape an unlimited number of times, so one option is to have your villain constantly shifting from one creature to another while harrying your players. With a high movement speed, it can seem like the players are being stalked by several creatures, rather than just one Druid.

Shapechange would kick this up a notch by expanding the options from "beasts" to anything other than undead and constructs.

Storm of Vengeance over a small village would increase the threat level as the players would have to beat the clock to avoid a massacre.

Mirage Arcane would let the villain effectively change the entire battlefield to suit their desires. Cut off the player's escape with a 1 mile radius circular crevasse, add in secret tunnels and paths that only the Druid knows about, etc...

Just Erratic Wildshape + Mirage Arcane + Investuture of the Wind(C) + Longstrider would make for a pretty terrifying encounter. Anything on top of that would be gravy.

Toss in Speak with Animals + optimized Animal Handling to give the villain an entourage of rabid beasts.

Oh man... I just realized that with unlimited Wildshape, the Druid could technically Wildshape as a bonus action every round and effectively have an infinite buffer of hitpoints, so you're going to need to be careful with that...

Thanks for the tips, and I have thought of the whole "infinite hp" scenario. Basically, instead of getting more hp everytime it shifts, It instead has a pool of hp and no matter what form it takes, the amount of damage taken in that form is taken from the pool.

Druid - 250 hp, Turns into bear - 34 hp, ranger shoots it - 24 hp, reckless wizard fireballs - bear is dead and now druid has 216 hp.

That probably makes the most sense.

The other thing you may want to do is consider "Lair Actions" and unique abilities. For example, maybe the Druid's madness is contagious through a bite or scratch, NPCs, animals, and players who get targeted might have to save against becoming rabid (Crown of Madness type effect). An Aura of Nature's Wrath could animate plants within 20ft to ensnare and attack anyone who gets close. 

For flavor, as much as anything, the Druid might be followed by an unnatual number of Ravens or Vultures who feast on the inevitable carnage.

I've never really thought about a druid as the BBG, but I love it. I was thinking it could be a druid of the spores, except with much much better stats because the Spore Circle absolutely sucks, and worship Zuggtmoy, demon queen of the shrooms and spores. The BBG could live in an abandoned mine where the mushrooms and spores grow free because of the exposed sulfur or other minerals. The druid could have lair actions to cause spores to explode somewhere. The druid can have innate spellcasting because of their ties to Zuggtmoy, such as Gaseous Form, but in a puff of spore clouds. Or Cloudkill a few times a day. Insect Plague is also fairly interesting. So if I were in your position, I would do something like this, the spore based druid who relies on acid damage. I also like Memnosyne's ideas of sending creatures to thwart the players. For a spore druid, you could use things like spore monsters, cultists of Zuggtmoy, or even lesser druids of the spores.