There is some debate as to what actually constitutes serial murder, but a common definition is as follows: A serial killer is an individual who kills 3 or more people in at least 3 separate incidents, with a cooling off period between the murders. This 'cooling off period' is a span of time during which the killer returns to apparent normality. He/she may go to work, take a spouse out for dinner, or socialize with friends. The important point is that he/she does not kill during this period. There is no real agreement as to how long the killer has to wait between murders for it to count as a cooling off period, but most would probably agree that we are talking about a few days or more (some serial killers can have years between murders, like the infamous BTK Killer who terrorized the residents of Wichita, Kansas, for more than 3 decades).
Approximately 90% of serial killers are male and about 65% of the victims are female. Serial killers tend to only target members of their own ethnic group (though there are notable exceptions, such as the Milwaukee Cannibal, Jeffrey Dahmer, a white male who mostly targeted African-American men and boys). Nearly half of all serial killers start in their twenties. The most common method of killing is strangulation, a relatively up close and personal method (compared with, say, shooting someone from a distance). However, female serial killers tend to prefer poisoning.
Whilst it is certainly true that a large number of serial killers have experienced substantial childhood trauma, this is not always the case. Some serial killers simply carry out their horrific crimes because they enjoy it. And plenty of people recover from childhood trauma to become productive members of society.
You might also be interested in:
Early Warning Signs of a Future Serial Killer, Organized vs Disorganized Serial Killers, The Six Motivations of a Serial Killer