Why are serial killers fascinating?

The fact that they can do what they do and walk amongst us masquerading as normal people is fascinating

What they do is pretty much inconceivable to the vast majority of people. So people are curious about how they can do what they do. People tend to be interested in things that are strange. It's also frightening and horrible, which gets some people's' attentions. It's the stuff of horror movies, but it's real.