What is dangerous is to let psychopaths that already committed crimes out of prison again, you can be 100 % sure that they will start committing the same crimes again the moment they are let into society and then in some cases they will escalate.
There are very good tools now to diagnose them, one being brain scans, and if they get this diagnosis and already committed crimes, I don't see the problem with keeping them in some kind of "security custody". In some European countries (maybe in most or all, I'm not sure) you can do this but only if they already committed murder (or, possibly, I'm not sure about that, committed crimes that show how much of a danger to society they are).
The thing is, let's say someone suffers from some kind of psychosis and they commit crimes, even murder because of their illness, they shouldn't even go to prison, they should be treated at a psychiatric hospital and have to stay there until they respond to treatment and then they are not a danger to society anymore and you can let them out. This is also what is done in a lot of countries, probably even in the US, not sure. But psychopathy is a kind of mental disorder (not an illness in that sense) that can't be treated. Just look at JJD and the hundreds, no, thousands of lives he destroyed, not just the victims lives, but the lives of their families, friends. Just one person. Had he been caught early, all this could have been prevented, all the suffering that this one man caused to thousands of people (and even more if you count the people who lived in the areas that he was active in and were scared to death for years and decades). If we have the tools now to prevent this, why not use them?
And yes, of course do more and more research to get more tools and better tools. There is a lot of research going on and a couple of years ago, just to give an example of one of the symptoms, if you will, is that they have a different speech pattern and they talk more about material things like food, sex and money than non-psychopaths.
http://news.cornell.edu/stories/2011/10/words-psychopaths-reveal-their-predatory-nature
Agree, and I tend to believe someone in LE when they refer to a suspect as a psychopath. LE has seen the incredible damage they cause and if they are able to catch them, they witness their lack of empathy for their victims as well as their lack of remorse for what they have done. I frequently watch Joe Kenda and he has stated that he still has nightmares about what he's seen. No doubt!