So sorry.
Now you need to focus on the rest of your flock. They still have a big problem. Put Oxine on your shopping list. It is a cleanser used to de-bug housing. Use it as directed to treat the coop. Clean your coop from top to bottom. I use a shop vac to remove bedding. It works great. I'd recommend vacuuming the interior of the coop out and then spraying Oxine on every surface, especially the roosts which is where the bugs tend to congregate when not on the birds. Some folks choose to use linseed oil on their roosts to prevent this kind of thing. I personally don't use it, but my roosts are painted so I don't need to. You need to dust the rest of your flock ASAP. When you put in their new bedding I'd liberally sprinkle some dust in there as well. You will need to repeat all this in 7-10 days, so go easy on the bedding. Then I would institute a bug check routine to make sure you don't have a repeat of this.
Mites are a nuisance. They're also rapidly fatal as you have discovered. Keep a close eye on the rest of the flock. Some Poly-Vi-Sol in their water would help immensely. Good luck.