I have always been fascinated by the serial killer in the 19th century with acronym JTR, Jack The Ripper. he was so sly that he had never been caught and it had always been a mystery.

I believe that serial killer is the most horrendous group in our society. they don't have the morality, however, they are smart in their brains. serial killer has always an event that triggered themselves to become a sociopath or simply a very-selective person (to choose victim with a common thread).
they commit the killing for the thrill of it-at first they maybe hesitant on what he/she was about to do, after the first kill, they always seem to have gain a lot of confidence. they want to mark their work, hence leaving clues that will link one crime scene to another. the link they think of-has been thoroughly thought as well as their MO. they want to make sure the forces wouldn't be mistaken his/her work to other criminal.
the killing they do is a like a masterpiece to them.
serial killer is the worst thing you ever wish for to be in your society. sometimes, serial killer lives well-off but commit the crime for the fun as others may have gone crazy or simply holding a grudge to a certain group of society.

perhaps, the adrenalin rush of blood through the vein is very thrilling and an excitement. they maybe caught and go to jail-may be sent to shrink for creating a new view of life for the perp. but one day, it will come back to haunt their life and the sense of remorse start to gradually come into heart(good news)- what if they miss the rush, the thrill- they might be doing it again.
thus, we need to always be optimistic so that we will not have the desire to do anything adrenalin/rush stuff that is against the law.