Dream analysis is always difficult. We often won't ever know the answer of why you had the dream. But I'll do my best to try to give you some idea of what you saw.
For most people, dreams are crazy and weird and is the brain's way of resolving real life's conscious challenges. But for people who have spiritual abilities, like empaths, dreams can also be lessons given to us by a whole variety of sources. And then there is astral travel where you normally control where you visit and how you interact in another world (whereas with a dream you're just there taking it in involuntarily as if watching a movie).
It doesn't sound like you were astral travelling since you didn't seem in control of the place traveled to or your actions once there. My best guess is that either your own mind is trying to help you better understand the thrill that narcissists get from hurting people, or this was a spiritual lesson sent to you from someone trying to help you (a guide etc...). Based on your other posts with your recent struggles with a narc at your office, this dream may have been an extra lesson to help you understand how narcissists operate, and why you must not let them overpower you and get close.
A very important trait for most empaths is that we have an overwhelming sense of justice as well as need to protect the innocent and defenseless. Your dream drew on both of these empath traits and got you upset. I think that was by design. I think the message was meant to upset and motivate you so that you will step forward and be the lightworker that you were meant to be and help protect innocents against those who live for the thrill of hurting and manipulating others (narcs, other bad people etc...). I know recently you have struggled with your gift and the anxiety that it brings you. But you will get that under control and eventually take the next step in your lifetime to using your gift helping others. That's where the lightworker analogy comes in.