Perhaps this doesn't generalize outside my city (Chicago), and perhaps data from the CPD is not to be trusted, but here the police claim that ~50% of murders are "gang-related" and 80% are "gang-associated" (i.e. either the suspect or victim is believed to be involved with gangs). The number of suspected gang members is not so high that you would expect 80% of murders to be "gang-associated" by chance (though it's not clear which way the causality goes, it seems highly plausible that people in gangs are more generally violent, but perhaps generally violent people also always join gains). There are also second order effects--more violence around you probably makes you feel more like violence is an acceptable solution?.

Note that applying 50% to the number of 2019 Chicago murders already saturates the nationwide "Gangland killings" in the FBI UCR, suggesting that perhaps many of the "unknown" murders would fall into that category. It would also not surprise me if many of the "other arguments" killings are also gang-related or at least gang-associated. Of course, it doesn't help that ~half of murders go unsolved here (and in many other cities) and gang-related violence is especially unlikely to find cooperative witnesses.

That said, this is only place with a police department not known for its honesty. Other places might be different. Southern states all have above average white homicide rates, but only Louisiana seems to have an above-average Black homicide rate, at least according to https://www.cnn.com/2018/04/23/health/gun-deaths-in-men-by-s... (caveat 1: this is victims, I could not quickly find corresponding data for offenders. caveat 2: most Blacks live in the South, it's not surprising those states might drive the average.).