You should be quoting violent crime rates rather than murder rates. The articles premise is that violent crime is infectious but the infection does not spread if the patients all die.
Othe crime stats can be easily and massively gamed.
What if there's a living person who would have died if they had not received prompt medical care?
https://www.umass.edu/newsoffice/article/research-finds-us-m...
When looking at trends, murder rate is a better metric than counting violent crimes, because statistics for other violence are quite easily impacted by changes in policies of how to record reports.
A dead person is a dead person, it's a very consistent metric. Of course, it is also a changing metric as emergency care improves - wounds that used to be deadly can now be treated. For instance, Swedish cities are reporting a vastly improved care for shooting wounds, as hospitals have gained experience and developed methods in treating them.