I seriously doubt any of us here admire somebody who repeatedly killed people. There is a big difference between being fascinated with serial killers and wanting to be one. My interest, part of a bigger study on crime in general, was perhaps a way to figure out the whys and hows of human beings. Often times, it's the strange cases, the extremes, that teach you the most; and a serial killer is nothing if not extreme. Average people can be malicious, can commit petty crimes, can even be downright cruel... Maybe part of our fascination with serial killers is wondering what would happen if we ourselves didn't rein in those tendencies, if we didn't have the good to balance out the bad. It's like stepping to the edge of a cliff and looking down, knowing just how easy it would be to step off the edge, and at the same time knowing you'd never do it... After all, it isn't so very hard to kill a human being; it's just our free will keeping us from doing it. Maybe that's part of it, too--the marvel of being able to choose.
I was also fascinated with the Holocaust--especially with the people who managed to smuggle Jews and other vulnerable folks out of the country, or else hid them; or the people who helped each other in concentration camps, despite it being dangerous to do so... I suppose that's the other end of the human spectrum, morally. Also, there's something decidedly attractive about tricking a huge, powerful war machine of a country to keep a few people alive. It took half the world to defeat Germany; and yet there were a few groups of people who somehow managed to resist the whole thing all by themselves.