Heh, your own descriptions of the research impeach it. "Acquaintance" of course includes e.g. drug dealers killing each other. They are generally "acquaintances" if not exactly friends, or friends any more; in general, stranger murders are relatively rare.

And your citation first mentions Arthur Kellerman, who's infamous for scoring undesired civilian deaths vs. civilians killing criminals, when such killing cannot legally be and is not the objective, stopping them is. There are many other problems with his "research", but when it's invalid at that high a level we hardly need to go into the details.