Hmm, you tank is over stocked so it could be stress leading to a) a normally unaggressive fish being violent and b) a normally predatory and robust fish getting bullied and being stressed enough to kill it.
The tank would be fully stocked with the gelius barbs, BNs and one other shoal of fish (6+ harlequins or black widows or ruby barbs). You've got several partial shoals (the 4 barbs/rasboras/tetras and 2 corys) - these fish need larger groups which could be generating a general sense of stress in the tank.
The shark will reach between 6 and 8 inches and needs a 40 gallon tank if you want to keep it in a community. They are also very aggressive as adults.
Nitrite needs to be zero. This could also be contributing to fish stress and thus uncharacteristic behaviour. Do you have a liquid test kit, or are you using strips?
I know these things don't seem related to your problem but think of it this way: in an appropriately stocked tank with excellent maintainence, appropriate water conditions, no toxins, plenty of hiding spaces/plants and the correct feeding, most people don't see sickness or bullying. If anything is out of sorts (in your case you have one unsuitable species and too many fish in your tank), the fish get stressed which leads to them becoming weaker, more likely to be bullied or more likely to bully and more likely to get sick.