@Ai9964 She's not a Yandere, she's just a confused but good girl that has emotions she wishes she didn't have.
@sakurabonx3 She's not actually a bad person. Humans are naturally flawed. And especially as a kid in love, she's being worse.
But the harder part is that she kinda has good points... so she can feed her negativity...
She's not against it for good reasons, but there are good reasons to be against it... so it kinda sucks for her.
It's a case where the reason she wants to stop it aren't good, but she has good reasons to stop it.
It's good reason feeding bad intention....
@Istvan90623 Her basic points are solid. They could make things weird for their parents; and as high schoolers, they are very likely to screw things up between themselves and could ruin their parents happiness based on their not so solid relationship.
And that's what is making her situation worse. She knows she's just being jealous and hates herself for it, but she can justify it because their relationship with each other probably isn't the best thing.
It'll be fine cause it's manga; but let's be real, many break ups aren't cause the couples are bad for each other, but because they don't deal with their problems well, or one person just does a major screw up and things break apart). And as teens/young adults, they are much more likely to do a stupid screw up than their parents will.
(I mean, if their parents fell for each other, objectively speaking, they would probably be a good match themselves.)
And because their relationship isn't the best thing, she has justification for the ugly emotions she hates and doesn't wish to have. It's basically feeding something she doesn't want fed.
It just reminds you that being a teenager can suck. You feel sh*t you wish you didn't feel, don't have the emotional maturity to deal with the problems and just have a hard time getting through with your issues.