Devlin became a Serial killer after he killed Sophie.
Devlin kept trying to recreate his time with Sophie, it was the only way to sate
his madness.

Upon meeting Mandy he knew she was Sophie's reincarnation. For while other woman
reminded him of his Sophie Mandy was the only one that glowed like her. She also
knew things. 

He is a very practicle person when it comes to supernatural events he doesn't believe
but Sophie changes everything.

(Spoiler) 
In order to get on Sophie's case he took out Brent Masser, a fellow collegues family.
While it appears at the end of the book he is upset with Devlin, he is not.
Brent is the reincarnation of Eyo and cared not for his family...not when it came
to Devlin. He is able to deduce it was Devlin and this only makes him more devoted
to him. He helps Devlin to fake his death, allowing him to freely move about in the 
shadows and make "arrangments."