The whole point of the chapter is to literally show you that she only does this to Shizuka because her dad is having an affair with Shizuka's mom, causing her mom to be a wreck, which the mom takes out on her, and she takes out on Shizuka.
No one's making excuses about anything, Marina is still wrong for bullying, it is literally just giving you the point that they're both in an environment that makes them like this.