I am so sorry; I know what it's like to have a predator -- in my case, a mink that killed three of four hens in a coop -- massacre your chickens.
I'm guessing that your Buff Orp was fatally bitten? I wasn't sure when I re-read your post, but it didn't sound good.
First off, a predator will ALWAYS return to the site of an easy meal or kill. Beef up security at your coop. Following the attack, I upgraded to hardware cloth EVERYWHERE and all of my coops now have security hooks and/or multiple latches.
Second, your surviving Australorp, even if she has no physical injuries, is likely to be traumatized for a long time. My surviving hen was spooked for weeks. I had the luxury of having other coops and other hens; I let her choose her own new home and coop mates. She eventually settled down and had a good quality of life for the remainder of her years.
Third, it's easy to blame yourself. But you didn't intentionally expose your beloved girls to danger.
I'm willing to bet that EVERYONE who has chickens has at some time -- or will in the future -- lose birds to predators, disease, weather-related issues, etc. It's impossible to protect them every minute of ever day. Now is the time to figure out how to reduce the likelihood of another attack.
Although I was unsuccessful in trapping the mink, I did set a live trap for several nights and slept on the sofa with the window open (in the middle of a frigid winter) so I would hear any problem in the coops. It was a Very Long Time before I was more comfortable about my birds' safety.
Take care of yourself.
