Can psychopathy be measured in the brain via FMRI, etc?

...would change their mind if they are to go to Nigeria

I think that's the difference. True clinical psychopathy would not change their mind. Their brain functions differently than most people (from what little I understand). Their mirror neurons (if those are a thing) don't fire the same way. All of this is a result of physical structures of the brain due to the nature/nurture inputs leading up to their current existence.

When we start throwing around words and accusations like, "He's crazy" or "What a psychopath" without understanding the professional use of those words, then we start to lose an important aspect of reality.

I'd prefer to talk about this in terms of the expanding circle of empathy. That, to me, is something we all share as part of our humanity, if we have healthy functioning brains.