The victim must not know the killer, it must appear to be completely random. The killer must feel that he has "dominated" the victim. The murders are completely psychologically driven, not for profit. Every kill must be symbolic to the killer. The victims are normally vulnerable targets to the killer. Statistically speaking the average serial killer is a white male from a lower to middle class background, usually in his twenties or thirties. A major question that will pop up among people is if a serial killer is insane or not. Simple put, the answer is no, not by legal standards that is. To quote Donald Lunde "The incidence of psychosis among murderers is no greater than the incidence of psychosis in the total population." A serial killer does understand what he is doing is wrong, and therefore, cannot be classified as legally insane. A serial killer will want to make himself seem insane though, at least those who are planning on getting caught will. If an individual can be classified as legally insane, they can avoid death row for their crimes. An example would be the case of the "Acid Bath Murderer" who drunk his own urine to prove that he was insane to the jury.
A major psychological abnormality of a serial killer is the development of an alter ego, referred to by psychologists as "alternate personality disorder". These "evil" sides are sometimes created to take the blame of the acts of a serial killer in the serial killers mind, or even in the court room. Most cases are in reality, not true multiple personality disorder, and are cracked and broken apart by trained professionals. A true case of an alternate ego in a serial killer is a rare find. Schizophrenia is also a widely common disease to be found in serial killers. Schizophrenia in itself sometimes drivers the people into their madness by hallucinations and voices in their heads. An excellent case is the case of Herbert Mullin, who believed he was stopping natural disasters by killing people due to the voice in his head.