Because it is prejudicial, which is unfair because it fails to take into account the person's individuality. Such traits do not simply equate to behaviour, the relationship is complex, incomplete and the mechanics not understood. A person's membership of a class which they neither chose to be in, nor can choose to remove themselves from (in this case a genetic brain anomaly), can not infer a fact about their mind or behaviour.
I don't doubt that while some "psychopaths" will be detectable ahead of time, and maybe mitigated (whether or not that's ethical), there may be many with "correct" views read as possibly psychopathic that will get strawmanned into oblivion due to their diagnosis.