That's right, Trek or Treat. But if you need serious...
Sylvia and Korob were so alien that humanoid mind and humanoid life were totally foreign to them. To understand and/or control, they attempted to probe the conscious minds of crew and instead reached the unconscious mind, where they were unable to determine what is human reality and what is human imagination.
They probed the basic primal unconscious human mind, elemental fear, which they manifest in symbols of horror and terror (skeletons, torture, ghosts, etc.)
Like most ST, it's not really a bad idea, maybe just failing in execution. And it's been seen many times in ST... aliens who seek to understand mankind and latch on to one aspect by which to judge us (and make an eps)