Green’s Dictionary of Slang

a cynical dismissal by police of a victim in black-on-black or, gang-vs-gang killings; also of those considered deserving of death.

2004		D. Grann ‘The Brand’ in New Yorker 16 Feb. 167/3: A convicted murderer was found strangled in his cell at a federal prison [...] Law-enforcement officials often dismiss such crimes as ‘N.H.I.s’—’No humans involved’—because the victims are considered to be as unsympathetic as the perps.
2020		D. Winslow ‘Broken’ in Broken 30: ‘Garbage taking out garbage [...] Even better when the garbage takes itself out. In any case, NHI’.