So much empathy here. We found out that one of our dogs was a killer when we moved to our new farm and got chickens. She mauled two chickens before finally mauling a third and killing three one day. Like you, I held the sweetest, darling little girl (a barred rock named Baby Bar) as she died in my arms. She was everybody's friend. Always came to get me when it was time to go in for the night. "Come on! It's bedtime!" (said sweetly). Had to walk with her to the coop door before she would go in, and if I left too soon, she would come and get me again. LOVED that darling.
The rage I felt toward that dog. Once Baby Bar was dead, and the other stabilized and being watched by my husband (who came home from work because I was so utterly devastated), I put the dog in the car and drove her to the no-kill shelter. I have volunteered there and wrote a grant for them also, so I was hoping they could help me find a home or a foster home... but they had moved. I decided to take her to the home we haven't sold yet (and are sometimes living in). She has been there ever since. She's lonely, and it's not the best situation for her. I have come to accept her, but it's not fair for her to be just tolerated.
I looks like my daughter and her boyfriend might rent the house and take on the dog as well. She's a very friendly dog, and the boyfriend is very good with dogs. If not, I am going to approach the shelter (I found their new location) and see if I can get her on a waiting list or something.
You've done the right thing. She will find a family that will love her. You will find that your farm is calmer, the other animals calm, you not constantly on the alert. Your chickens will probably be healthier and happier without the stress.