the difference is that the scavengers eats those which was left by other animals or which may have died from illness or old age they use their canines to tear the flesh but the decomposers are mostly microorganism that doesnt have canines such as saprophites(produces enzymes to digest the food externally then vacuum it such as fungi etc) or you may just put it this way scavengers are macro eaters while decomposers are micro eaters
My perspectiv to decomposers and scavangers would be this. Decomposers are those organisms that break down organic material to its basic components in sense that very few organisms can eat and live on their leftovers and excrements. In that gorup I would put bacteria and fungi. Scavangers would be organisms that activly search for dead organisms no matter of their size so everything from small insect to hyena.