What other types of fish are in the tank now? do you have any other platies? if not, you may consider getting some, preferably of the oppisite sex of your current platy. the fish you already added might not have been attacted, they might have jsut died from being stressed by the bully. stress is very bad for fish.
worst case senario, the bully platy will have to live alone. i have a isolate tank, and at one point it was the home to one bully swordtail and one bully loach. since they were both bullies, they didnt really fight with eachother.