I would love to think that we are capable of eliminating mental illness in adults, or at least diagnosing it early enough so that the harm that individual is capable of is limited, but that's the problem with true sociopaths, they fit in, can pass the tests and are completely unsuspected of wrongdoing until they make a mistake and get caught. And even if we do eliminate the 'threat' of mental illness, what about clinically 'sane' people who seek the mass destruction of others for some personal or political goal?

Personally, I hope that by the time nuclear-scale energy densities and home bio-printers become prolific we are spread out enough as a species (Hopefully off planet, and even more hopefully on our way out of the solar system) that one lunatic can't kill more than small percentages of populations. A possible far future that I see is one where you live in physical proximity only to your immediate family and possibly friends, with most interaction beyond those groups occuring in cyberspace. The hope being that we colonize enough heavenly bodies to give everyone their own refuge in the middle of large tracts of wilderness, with cross-planet and interplanetary travel being as simple and commonplace as automotive commuting. That being said, I fear we are heading more towards a "The Moon is a Harsh Mistress" or "The Forever War" type of future rather than a late stage "I, Robot" kind.