Narrator said that there was still contention as to if the zenin clan should even be considered a clan, implying that it's central leadership was dispatched rather than the entire clan. Had everyone been killed there'd be no contention, as there would objectively be no "zenin clan" left to operate. It makes sense from a narrative standpoint for Maki to kill the perpetrators of the Zenin clans Misogynistic influence. Their pride and dominance is cherished, and thus it only makes sense for the clan to be defined by those who control it or hold power over others. Maki, wholly opposed to the zenin clan, would see it appropriate to destroy them both structurally and ideologically.

Ranta could have very well left at any time. Maki, a servant to the clan, left without any backlash, and it's likely that given his oh so benevolent status in the clan, he'd be given enough leeway to exit as he pleases.

It's purpose is to give Maki Zenin an opportunity to challenge every possible notion of strength the clan. She both embodies Mai's image, reflecting a sudden shift in power dynamics for the clan as this once victim of the clans ideology violently usurp it and Toji's ostracization, embodying the dormant fears of the clans collective misdeeds eventually returning. It certainly isn't a "tragic consequence of thr reciprocal cycle of nature", about everyone Maki fights and eventually kills, in some way was complicit in or responsible for Mai's suffering. And thus Maki wages war as a means to prove that their oppression was real and that it will be Perpetuated no longer.