The idea of mass hysteria is very frightening. One person starts to believe something, like a girl in Salem claiming to be possessed, and then it spreads like wildfire. Everyone becomes paranoid. And the fact that it was often the vulnerable, like old women or those on the fringes of society, who were targeted makes these stories even more disturbing.
The false accusations are a big part. Innocent people, like in Salem, were just living their lives when suddenly they were accused of being witches. It's scary to think how easily a community can turn on its own members.
The torture methods used during the trials are horrifying. In many places, accused witches were tortured until they confessed. This shows the inhumane side of those times. Also, the superstition and lack of scientific knowledge that led to these trials. People really believed in witches and the devil, and that made them see evil everywhere, which is quite chilling.