I've always thought of "Zangyaku/Brutal" as a catchall term for Choujin that are, well, brutal and bloodthirsty, but not associated with the Devils or Perfects.
The dMp was formed as a means of survival and future revival for non-Seigi/Justice Choujin rather than out of any unifying ideals, and as we know, a faction war lead to the near-wholesale destruction of the group. Sunshine mentions that the Devils were the fewest and thus were given the least space (and, presumably influence) in their headquarters, but since none of the dMp members shown display any signs of being Perfects, I assume them to have been Brutals.
Traditionally, I categorized the three camps as:
Zangyaku/Brutal: Chaotic Evil/Neutral Evil. Selfish and sadistic, but - pre-dMp - ultimately ignorable since they rarely pool their resources or have any ambitions outside "kill opponents and show off." Some limit their victims to opponents in the ring (pre-Face turn Ilioukhine/Turbinski, Destruction) and are technically "Neutral Choujin", others don't (pre-Face turn No Respect).
Akuma/Devil: Neutral Evil/Lawful Evil. Serve Akuma Shogun and/or Satan. Dangerous since they usually work together towards enslaving humanity, but occasionally show a sense of honour, as in the Nisei finale. In this new arc, they are depicted as the lesser evil, which I find strange and somewhat aggravating.
Kanpeki/Perfect: Lawful Evil. Haughtly elitists and Social Darwinists. Arguably the most threatening foes pound-for-pound, but relatively few in number. Some characters act like Perfects in all but name, such as Super Phoenix and - at times - even Kevin Mask and Mars. In this arc, they are presented as the Big Bads.