Shimei: Huh. You humans are nothing but chess pieces for my enjoyment. Like you, little miss (flashback to the deaths of all the people Shimei has killed-Kouei (Housei’s master, Laila’s friend), Laila’s father, and Housei)- and all those other dead losers.
(Laila’s eyes are empty and horrified.)
Shimei: Your life rests in the hands of-
Laila: …will never…
Laila: I’LL NEVER (she grabs her knife) FORGIVE YOU! (she lunges)
Shimei (suddenly behind her): So what? (grabs her ponytail) Shut up and just hand over your body.
I really love that these flashbacks show so clearly that Shimei was never Taito’s villian, but Laila’s. Laila is the one Shimei has hurt the most, who has taken so many things from her. Shimei nearly killed her the very first time he showed up. Shimei killed her friend. Shimei killed her father in front of her and rubbed all her loss in her face. Shimei manipulated her (possibly with sorcery) into attacking her brother as a result, and it was her strength that let her shake that off. Shimei tried to kill her again, and then killed Housei right in front of her. Taito wasn’t there for his father’s death, that was entirely Laila’s story (and the anime fucked that up. We’ll see if they fuck this up too).
Shimei has victimized Laila and Laila’s friends so many times, and she hasn’t been able to do anything about it. So of course she must be the one to face Shimei, to have this final battle, to avenge what she’s lost. Most narratives wouldn’t think of it this way because women exist to be victimized by the villian, why should their pain matter, why should they have an arc and closure there? In most narratives, the fact Shimei actually hurt Laila a lot more than Taito wouldn’t ever be noticed. But this is Hiromu Arakawa, and she gives her ladies the due they deserve.
I also think it’s really important that Laila is fighting this battle to protect someone, though, not to avenge anything. She has a little boy who’s dependent on her and she knows if Shimei takes her body, it will be used to kill her brother and other friends. It was mentioned earlier in the manga that Taito can only fight to his full potential when protecting someone, and it’s the same for Laila. The reason she couldn’t take on Shimei before, that she’s coming into her own now, is because she needed someone the protect to truly fight to her full potential.