Keep in mind that there is no loving romantic relationship to be had with a psychopath.
While most people believe that the psychopath lacks the capacity to form an attachment with anyone, this isn’t exactly true.
Actually, some psychopaths do have an attachment to at least one person, and that person is often a mother or a grandparent who was loving and kind.
Above twenty traits can be measured on a 3 point scale and the cumulative sum can be used to characterize a person's psyche..
There are some other methods too like Psychopathic Personality Inventory, Diagnostics and Statistical Manual of mental disorder( DSM) etc.
It would be hard to answer this question accurately because psychopaths are experts in disguise, and it can take a long time – months or even years – to see the full extent of the individual’s pathology.
While narcissists are oriented around establishing themselves as superior in every situation, the psychopath is focused on having power over others and exploiting others to get their needs met.
In general, the psychopath is far more dangerous than the narcissist because the psychopath doesn’t form true attachments and he feels no remorse, even for the most heinous, injurious behaviors.