The original comment said "Detroit has more murders than New York, Los Angeles and Chicago combined".
They also said "Detroit, the city with the most violent crime per capita in the US"
These statements are false. Chicago has more murders than Detroit. St. Louis has a higher murder rate than Detroit. The person you are commenting on is correct.
A quick search resulted in these statistics from 215:
https://www.statista.com/statistics/718903/murder-rate-in-us...
* Detroit is 3rd on the list with 38.88 murders per 100.000 residents
* Chicago is 9th on the list with 20.7 per 100.000 residents.
Therefore at least according to this data Detroit's homicide rate is roughly twice that of Chicago.