Because it works in general? Even the worst country in Europe [1] (Latvia 4.9 murders/100k pop.) by murder rate is better than US average (6.6/100K) [2] with France 1.1, Germany 0.9, etc. So unless you choose extreme in Europe you are looking for sure at least at 3-5 times lower murder rates in Europe.
[1] https://www.statista.com/statistics/1268504/homicide-rate-eu...
[2] https://en.wikipedia.org/wiki/List_of_U.S._states_and_territ...