As for charming, do you mean their actual charm, or the very concept of psychopaths themselves?
They are socially charming due to lacking empathy and fear, so they can get to what they have to do to achieve or acquire something. If you think about it, there are many things humans can do without having to employ empathy- it may be that way for psychopaths. We smile when someone else smiles, we feel pain when someone else feels pain. We do not treat objects the same way. We're used to seeing living, sentient beings as things worthy of or requiring empathy, it is a given; while psychopaths just do not.
The concept of psychopaths may be charming because a psychopath is the embodiment of everything people want to do, but are just too afraid to.
Psychopaths do not feel fear. They don't feel the pressure of conformity, nor do they particularly require the approval of others as a need.
If we did not feel the impact of consequences, be it pain, fear, or shame, we'd probably do a lot more.
The separation is a very thin line, at the basic level. The only thing limiting a psychopath is their ability to get away with or to bear with the consequences of whatever they do.
Why are either of those that attractive? I do not know. Maybe at some deep level, we just wish for that kind of freedom.