Why shouldn't people diagnosed as psychopaths be separated and treated even if they have not yet committed a crime?
There are two major reasons for this.
First, not all people who are psychopathic will commit crimes at any point in their lives. They may be more likely to commit crimes, but there is no absolute guarantee that they will do so. For this reason, they should not all be separated from society.
Second, our legal system is based on the idea that we are only responsible for what we do, not for what we might do. We do have the crime of attempt, but even that only applies to people who have taken concrete steps to try to bring a crime about. We do not prosecute people for their thoughts or for their inclinations.
Since not all psychopaths commit crimes, and since we do not prosecute people for what they might do, we should not separate all psychopaths from society.