No honor among thieves. If the cops had joined the Cossacks, and started or provoked the altercation, those points do not rule out the cops targeting Cossacks, and may even tend to rule it in, as those Cossack individuals were known to the shooters.
I’d add that Cossacks who were unaware the O’s little friend was a cop, and “just figured it out,” may have been perceived as a bigger threat on the speculation that they may have thought they’d been betrayed or set-up.
Dead men tell no tales.