You're right that we don't have data on the incidence of psychopathy in ancient Rome, and our data its current incidence is pretty sketchy. (Unlike most mental illnesses, psychopathy is more a problem for other people than the person who has it, so psychopaths have no reason to get treatment. Not that we really have any treatment if they did.)

But there seem to be both genetic and social components (e.g. being abused as a child), so probably those same genetic opportunities got triggered in some people throughout history. Possibly at different rates than here and now.