We all have had to learn things about chickens the hard way from time to time. Don't beat yourself up over it, because now this is something that you will never allow to happen to one of your chickens.

It is so hard to lose one of our precious girls, and trust me, we always feel responsible no matter what the cause. Remember that you are giving your chickens a life better than they would have in the wild and better than they would have in a battery.
I've posted this so many times in the last few months, but I lost a few to eagles. In 4 years the eagles never bothered anyone's chickens around here. This year they attacked all of our chickens.

Now I have bird netting over the run...why didn't I do that before? Because I didn't see the risk. It's how we learn. We just can't know every question we should ask. My point is, you wont lose a chicken to mites again, and I wont lose one to eagles again. And because of BYC, our shared experience might help us not all make the same mistakes.
Hope I was helpful. I know you are sad.