You probably haven't had too many responses to this because it has come up in the past, and we end up fighting with each other!
You can probably find some of the other posts if you do a search for them.
Some folks feel that dogs can never be trusted with chickens, some can't imagine a flock without a dog's protection, and others feel it is up to the individual dog.
If you want my most humble opinion, I think it is up to the individual dog, and the time and effort the owner is willing to invest in training. Contrary to what many say, old dogs such as yours can most certainly be taught to behave around chickens. I always adopt older dogs from the shelter, and have always been able to train them to behave while the girls are free-ranging.
If the dog has actually made a kill, I wouldn't trust it alone with the chickens ever again.
Just a quickie note: it is a current fad for a lot of folks to get "livestock guard dogs" to protect the animals on their small farms. This is a dandy idea, but when these dogs are used to protect poultry, they need a little extra training to be gentle, especially with chicks. I have known people who have lost birds due to them being carried around or over-enthusiastically herded by one of these dogs.