An instinct, or compulsion, found throughout the animal kingdom is to prey on animals that display fear. Animals tend to experience fear in the presence of danger, such as predators. Therefore predators can use it as a signal to identify prey.
From a predatory viewpoint, animals display fear because they are vulnerable. Fear can suggest the animal sees itself as weaker than the predator — perhaps because it is hiding an injury, aware of a weakness it believes can be exploited, or just doesn’t believe itself to be able to successfully defend itself, for whatever reason. So, when a predatory animal sees fear in another animal, it will likely act towards it as prey.
In the absence of fear, predators start from the cautious assumption that an animal may be a formidable challenge, is not vulnerable to being attacked, or may be a threat. That it may have hidden strengths. It wards them off.
This predatory guidance system is a simple one, yet it extends from the animal kingdom deep into human behaviour, throughout civilisation. The instinct to prey on those displaying fear in many ways governs the modern world, and ends up dictating its power structures.