Why are serial killers fascinating?

Maybe because the neural connections in their brains are so completely alien to ours.

It obviously isn't just you, as some serial killers are still talked and theorized about to this day. As to why, it's probably just because they're different from the norm, not boring like the rest of us, same reason anyone's famous really

Most people want to know "why?' when it comes to a mystery. Trying to decipher the actions of a monster really brings out their curiosity to answer the question.

they are like aliens who walk among us.

Richard Kuklinski Is the scariest killer I’ve ever heard about because he was a hit man who had no emotion about killing for a contract hit but would also kill just for the hell of it.

He did not fit the bill of the normal serial killer who killed for the gratification of the act. Very scary man.

They're extreme. They don't ask permission and they color outside the lines.

Same with gangsters.

Part of it is that most of them manage to live very normal lives and hiding their true selves.

Fascinating? Serial killers are deranged people who have a grudge against life in general. That's why some commit suicide.