Mine are horrendous, they're permanently fighting. No idea why yours died though, what else is in there with them? How long's the tank been running? Params etc...
i had a pair of rosy barbs and they didnt fight amongst each other thought they seemed to be the favourite victims of my rainbow sharks bullying. they didnt seem to fight amongst each other. i had gotten them before my tank had cycled completely and almost a year on they dropped dead within a week of each other. was your tank cycled when you got your barbs?