Because you had too many mean fish in a teeny tiny tank. If you put three gangsters from downtown Brooklyn into a phone booth and locked it, how many would still be alive in there the next morning? One. Because they all want what little space there is to their own selves. And if you keep putting more gangsters in there, just guess how many pounds of earth you will have to move to dispose of the bodies and quit while you're ahead. No more fish.
If I were you, I'd keep that vigorous oscar and keep him alone in your 100g once it arrives. You'll love him like a puppy. Oscars are great, man. One is all you'll need. Happy fishkeeping.