Exactly. Prey animals are driven by nature not to show signs of weakness because to do so is to flash a big neon sign that says, "Animal in Distress, Eat Me." You are not a horrible human being. You could not have seen what she did not reveal. And as soon as she could not hide her illness, you jumped on it and took heroic measures to try to save her when many another might have culled her instead. No, your courage and compassion are an inspiration. Give yourself credit, and celebrate her life.Don't beat yourself up. Ducks hide ailments very very well and often don't show signs until it's causing them a great deal of pain or discomfort. Even the vet didn't know for sure what was going on and they're the professional on the matter. You took her to the vet when you noticed something was off and that's more then what a lot of people will do for their ducks. Your ducks are lucky to have someone who is willing to go the extra mile for them