That's the oddest thing. I have about 30 of them - some protective moms/dads, young birds, etc - living alongside geese, chickens, Welsh Harlequin ducks, and guineas. Not a mean bone in their body. Are you sure it was the muscovies? I'm not even sure HOW they'd kill chickens. I've seen a past drake get in a fight with a rooster, but it quickly quieted down, no one was hurt. They even share feed. In general, if a hen pecks a hen/drake they make a startled quack noise and scuttle away.
Of course, my birds free range which is less stressful than a penned situation, especially with new arrivals. Lessens fighting dramatically.