Poor poor ducky!

So we have 5 year old hens and just got 6 new chicks and 2 ducklings about a month ago. They all share a coop. We've had to separate the ducks because the chickens were pecking their eyes. Then my husband thought it'd be ok to put them back together and we left for one night. He came back today and one of the ducklings had been attacked - both eyes pecked out and it's head split open with part of its brain showing. And it was still alive!!! My husband had to shoot the poor thing. And one of our adult chickens is covered in blood and won't move its leg. I don't know what happened while we were gone but I'm traumatized and can't stand to see anything else happen to our remaining birds. Will the duck be ok by itself if we keep him separate from the other birds? Or without his buddy is he going to be too upset? They were bought together and attached at the hip. What do I do about the hurt chicken? I am just so sad this happened. We can't tell if it was a chicken attack or if some predator got in and attacked, but even that would be difficult as the coop is pretty secure. Thoughts?? Suggestions??