Add another one to the list of "the protagonist is actually the monster" horror stories, a list that starts as far back as Frankenstein. Both patients are ultimately unfortunate victims, and despite their terrifying appearance, the real horror lies with the doctor, who administers the crystals to new victims. To be sure, the concept of a long dream broken by small gaps of a waking world does not necessarily have to be horrifying—it's actually a pretty famous thought experiment known as the "experience machine". What makes it horrifying, then, is the fact that the dreams are nightmares. The first patient says it pretty clearly himself, "Had they all been pleasant dreams, I wouldn't have minded". By that thought, the doctor's logic perhaps isn't wrong. But clearly, the dreams are mainly nightmares, and that in turn means the doctor is a monster, both for harming his patients and for imposing a philosophy upon them that they may not hold.