Can someone be so empathetic that they understand and relate with a psychopath?

I would say generally not, as empathizers connect with the emotion of the other person, and lack of emotional responses is one of the key aspects of being a psychopath.

Of course. There are people on this very site that do that willingly.

Yes I like Alice from Luther and the guy from YOU

But they’re both bad people 🤭

Empathy does not mean relating. It means empathizing. Understanding. Grasping the other persons emotions and feelings. Empathizing with a psychopath to an extent, yes. Fully? Likely not.