[@MatthiasAngel] Serial killers are generally extremely intelligent, and sociopaths. This means they are unable to empathize with their victims' deaths. So his/her whole life, s/he's been unable to to feel remorse or anything along those lines. So s/he could be and act like a normal citizen while having serial killer tendencies, meaning s/he could have never committed a crime before and then easily go and kill the girl because they lack remorse. Also another common trait of serial killers is that they like to take "trophies" from the victim's body, whether its jewelry, shoes, etc. So I can see it being reasonable that s/he would take a chunk of the body as a "trophy."