Usually it's shown to let you know the character is pensive and/or stressed. People do this in real life (Although not to the point of fingers bleeding) like my brother. I have a feeling post/near end of this arc we're gonna get some sob story about her to try to make us feel bad for the genocider as per usual for manga.
Well that was fun enough. Curious though, I've seen a lot of villains but never have I wanted to bite one so badly. I hope the plan takes her down several notches.
it's been 3000 years......, it doesn't help that this chapter is really drawn, it was full of filler with that stupid cliffhanger ending, they had enough pages to simply start the battle