Bare butt chicken dead :-( Can someone please help identify the cause?

[echocannes]

https://imgur.com/a/vB4t9Gq

We found one of our hens dead inside the coop this morning. Her eyes were closed. She had a mild bare butt that we thought might have been lice, so a month ago, we dusted the chickens with Sevin dust and they seemed to be doing fine. We were very heartbroken to find one of our hens dead this morning and we want to prevent this from happening to the rest of our flock. Her butt turned green and there looked to be some mustard-brown "pus" coming out. Thanks in advance for any insight.

 

[azygous]

Of course we can't tell you what killed your hen. We can only guess. Only a necrospy. like a human autopsy, will determine cause of death by taking the chicken apart. You can also do that if you wish. Cut her open and see what you see.

Often you will find pea size cancer tumors floating in abdominal fluids. You might find and accumulation of eggs in the abdominal cavity indicating internal laying. You might find a liver that weighs over a pound rather than a few ounces as would be normal.

From the yellow fluid you found in her vent, she may have died from internal laying.

 

[Lemon-Drop]

https://imgur.com/a/vB4t9Gq

We found one of our hens dead inside the coop this morning. Her eyes were closed. She had a mild bare butt that we thought might have been lice, so a month ago, we dusted the chickens with Sevin dust and they seemed to be doing fine. We were very heartbroken to find one of our hens dead this morning and we want to prevent this from happening to the rest of our flock. Her butt turned green and there looked to be some mustard-brown "pus" coming out. Thanks in advance for any insight.

I'm so sorry for your loss.

As Azygous said, then only way to really tell is a necropsy.

 

[Bocktobery 10]

https://imgur.com/a/vB4t9Gq

We found one of our hens dead inside the coop this morning. Her eyes were closed. She had a mild bare butt that we thought might have been lice, so a month ago, we dusted the chickens with Sevin dust and they seemed to be doing fine. We were very heartbroken to find one of our hens dead this morning and we want to prevent this from happening to the rest of our flock. Her butt turned green and there looked to be some mustard-brown "pus" coming out. Thanks in advance for any insight.

So she plucked the feathers out herself, right? It does indicate irritation of sorts, something bothering her in that region, and yes, mites would seem a logical conclusion.
Mustard-brown pus and green tell me infection. Mustard brown- brown being old blood, bloody mucus. Was she eating ok? Green poop often means a chicken hasn’t been eating, and the green abdomen could mean it’s just bile poop (green poop) that one can see through skin from the outside. Small intestines would be near that area.
Do you freerange your flock? Could she have been kicked hard or smacked in that area? The green looks black and blue as a bruise. If she was kicked or got hurt badly in that area, it’s possible it resulted in an overnight death. But that doesn’t explain the no feathers unless it was mites which were taken care of with the Sevin dust. The more logical thing might be she was egg bound? Did she lay for you the day before her overnight death? If so, it’s not that.
As someone else suggested, you could cut and have a look inside and you can post a picture here to help identify. I do want to warn you that it’s important to know personal safety when doing a necropsy as you can get infected yourself. Then again, lots of farmers have been cleaning out their dead livestock for dinner for centuries, but its likely most of those birds were healthy when slaughtered. A necropsy from a skilled knowledgable person would be your best bet.

I’m sorry for your loss.