The ferret should be vaccinated, ours were anyway. But the owners sound like they lack in the most basic care, so probably not.
Ferrets have been used for hunting rabbits. They can be killers. My trio was sweet as can be. They never went after our pet birds, cats, dogs, or kids. But....they can. And I’ve heard many stories of them killing other pets and even eating off noses, lips, fingers, ect. Of little infants when the ferret escapes the cage at night and gets in the babies room. Some places have tried(I think some succeeded) in banning them for that reason.
You have every right to be upset. If it returns give it to animal control like other said. I can’t say to kill it. I know I wouldn’t be able to. I have a soft spot for them. But no one can blame you if you did. I’m so sorry for your losses
Ferrets have been used for hunting rabbits. They can be killers. My trio was sweet as can be. They never went after our pet birds, cats, dogs, or kids. But....they can. And I’ve heard many stories of them killing other pets and even eating off noses, lips, fingers, ect. Of little infants when the ferret escapes the cage at night and gets in the babies room. Some places have tried(I think some succeeded) in banning them for that reason.
You have every right to be upset. If it returns give it to animal control like other said. I can’t say to kill it. I know I wouldn’t be able to. I have a soft spot for them. But no one can blame you if you did. I’m so sorry for your losses

Good luck with that, will pray that the ferret won't be back, but be prepared if they are. There's plenty of good ideas on this thread. As others have said, would not let your dog attack it cause the ferret could have rabbies. If you need to dispatch it quickly to prevent it from killing more of your flock, opt to shoot it instead. You could also trap it again, just don't bring it back to those same owners! If you can't get animal control to take it, could try and rehome it yourself, or kill it if need be, I don't think it's owners will miss it, and they shouldn't have it.