Can I save my sick duck.

One of my muscovy ducks is unwell. I noticed yesterday its feathers were looking very weak. Its previous owners clipped its wings so the feathers always looked a bit horrible. After picking it up yesterday I noticed it had a sore around 5cm in diameter under its wing. It looked infected and has no feathers in it. It was getting around ok so I was going to get something antibacterial to put on today. This morning I have checked on it and the infectedlooking area is across its whole back under both wings. It looks very infected, it is bleeding and pus filled. The duck seems sick and in pain. I dont know what to do. Can I save it? Is it kinder to put it out of its misery. It is still waddling around and eating. I can add pictures if needed.