While I admit thinking too hard about Helnik makes me feel iffy, I do think their relationship is a part of a bigger idea Matthias repersents.
Matthias is an explicitly clear example of how a traumatized and hurt person (especially an impressionable and extra vulnerable child) can be manipulated into a hateful being. But he also demonstrates that bigots are not beyond hope, people are capable of rejecting hate.
Note this doesn't mean every hateful person is capable of change nor saying it's marginalized people's responsibility to facilitate that change. Nina doesn't do the work for him like some people like to imply. She was the push he needed and the thing that kept him going in moments of doubt. His deconstructing is difficult and often miserable. The extremes he feels caught between tear him apart.
His story is important even if most of us don't find it relatable, because people like him do exist in real life. It feels bad faith to call his character or Helnik a total glorification of bigots and the oppressor/oppressed trope. If that's how you interpret it that is you own takeaway, but not the only correct one.
