Chaos is like a force of nature. It is disorder and discord. Amongst humans, with our desire for reason and harmony, chaos is about the most disturbing force.
All atrocities are "senseless". They "boggle the mind". That is because they are chaotic, and thus defy logic or reason. And when people commit such atrocities, they are acting as agents of chaos. They have disrupted society and shattered the semblance of order that people had in their minds.
The thing that makes chaos both terrifying and fascinating is that it has no desires. It isn't trying to gain power or create political change. It doesn't want wealth or fame. Chaos simply acts. It does whatever it feels based on whatever whim enters its mind. Chaos is kind of the epitome of being a colossal dick.