In any case, the directives this minicollective follows have significant differences from standard borg imperatives - this was proven by the starfleet ship NOT being assimilated at the end.
Now that you mention it, it probably was
Section 31 who contracted Carson. The Starfleet ship was not assimilated, and note that no Starfleet members were harmed in the operation - only private mercenaries, people that Section 31 probably consider disposable scum.
You assume that the minicollective follows the directives they were intended to follow by the ones responsible for their creation - S31 or someone else.
However, the end of the story strongly implies that this is not the case - this minicollective seems to be out for revenge against whomever created it (definitely NOT part of S31/someone else's plan or implanted directories).