In Iceland, violent crime pretty much doesn't exist. According to the BBC, a stranger might randomly give you a ride, and you can be just about certain that nothing bad would come of hitching a ride with someone you met mere seconds ago. Not a lot of countries can boast something like that.
As for why, there might be some reasons. Economic disparity is almost nonexistent, with 97% of the population identifying as, essentially, middle-class, and everyone having access to the same education systems and social welfare. A lot of locals attribute the lack of crime to the focus on equality. Plus, guns are hard to come by, and hard drug use is pretty negligible. Then, when problems do start to arise, the government will usually take preemptive steps to mitigate the effects early.
Granted, this is all in reference to general crime rates, not serial killers. Low crime rates don't necessarily cause low numbers of serial killers, but the reasons could potentially be similar.
The country's only reported serial killer — Axlar-Björn — dates to the 1500s (via Iceland Magazine). He claimed dead bodies would just appear on his land, which he would then bury, and no accusations could stick because of his protection from a rich friend. He was finally caught and executed in 1596 after confessing to 18 murders (though most people assume he was responsible for far more than that).
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