Soooo sorry for your loss, but please don't be so hard on yourself! Your not a bad mommy. You have now learned to arm yourself from the cruel ways of nature. Luckily you are smarter than every single predator out there - now go work on provin' it dear. Every hole, every crack, no matter how small - fix it! That's part of chicken keeping, you get to play in the dirt and learn how to use tools! Get busy...then start over!
PS- Girls best friend, zip ties!
PS- Girls best friend, zip ties!