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.
As we can see by comparing the county map of the USA that you linked with a demographic map, murder in the USA is overwhelmingly a demographic issue. We can debate the reasons why, but the data itself is readily available and incontrovertible. The changing demographics of Western Europe will prove an interesting new data point for that debate.
What are the arguments against your position that you think are strongest and why are they so weak that you think they can be considered ‘incontrovertible’?
Fair objection. I thought it was obvious that the map was just a visualization and not the incontrovertible data. That would be the data from various high quality reports such as those produced by the UCR[1], BJS[2], NACJD[3], and the MAP[4].
I don't know of any non-straw arguments against my position that demographics are a dominating factor in homicide rates because of the high quality of the homicide data. Note that I say factor and not cause. The causes for these observable patterns are very much under debate. Nevertheless, corpses are hard to ignore. The paperwork almost always gets filed, so the data quality is considerably higher than for underreported crimes such as rape.
Yes. However I assume you mean they do at a per capita rate higher than people who aren't in cities. That's not necessarily true. San Diego for example has an admirably low murder rate for a large US city and San Jose isn't too far behind.
[1] https://www.statista.com/statistics/1268504/homicide-rate-eu...
[2] https://en.wikipedia.org/wiki/List_of_U.S._states_and_territ...