There's a million ways coops fail and result in chickens getting killed. Don't keep beating yourself up over something you couldn't prevent!
I rescued this hen a couple years ago after her owners dogs killed her flockmates.She flew on top of my brothers house to escape from the dogs so I climbed up and got her down and carried her home with me
Like other posters, I'm truly sorry this happened to you.
You state that you don't think you can keep chickens again. This is a normal reaction to losing almost an entire flock. I hope you change your mind. The one remaining hen would probably appreciate companionship, as chickens are flock animals, and are happiest when they are not alone. Also, you sound like a caring and loving chicken person. Any chickens owned by you will be lucky birds indeed.