Hole In Belly Barb

That looks horrible. However, one distinct possibility is that the hole happened after death. Fish will eat dead fish, and the belly is the softest part. I suspect the damage was caused by the other fish starting to eat it after it died.

But as for what the fish died of, it is very hard to say 

So sorry to hear about your barb. 

It's probably too late but I would have opened up the dead fish to see what was going on. Fish can carry parasites that can erupt through the skin but it is fairly uncommon and the fish would have been unwell beforehand. But like Sue says fish will eat dead fish and its very hard to tell from the picture.

Well at least that's something. I've got both eyes on my tank as much as possible just now with one cherry barb having finrot so I share your apprehension at having a potentially sick aquarium. Best of luck