This is either the best, most thought-out writing I've seen for an antagonist I've read in a while, or they somehow accidentally stumbled ass-backwards into a concept that your antagonists don't need to be "good, from their point of view, could be seen as good by the reader"
As long as they never explain WHY the bangles hold Lavi back from progressing further than his sister, or if the reason is entirely selfish, then Connie having every opportunity to learn to value those weaker than her but ignoring that to stick to her "The weak souls obey the strong" mentality works very very well.
Showing her as a kid beating up other kids for being weak wasn't to show her growth after meeting Lavi, it was too drive home that she was always a terrible person, but her bro-con-ness hid it from us.
She loves her brother because she thinks he's cute, but still thinks he's beneath her. And I love that in an antagonist. Thoroughly unredeemable, and they don't try to make her redeemable.