Insecurities. That is the wedge that was driven between them. Everyone has them. They are perfectly natural, but when they start infecting and dictating what you do and how you act, things get bad. They get real bad.
Many people might view it as Laura being self-centered in comparison to Julian who put his heart on the line and was lost. That's not the case. Julian has always been lost in himself and been self-centered. He's always let his insecurities dictate his choices, trying to gain more power, and used others to put himself above someone. He's a bully through and through. While the only reason he got the power boost was to save Laura, he's never shown any real redeeming qualities. Even that act he only did to repay her for the times she's saved him and in many ways had used it in his mind as to why they should be together. He has no self-less nature, he's entirely controled by his insecurities and his self-centeredness. He's the kind of boy who feels he's owed for every good deed he does. That's the problem here too. He's a boy. A litle boy trapped in the body of a grown up with grown up issues to cope with. The first chance he got he goes back to such juvenile antics and bullying of other students to make himself feel better. This just is who Hellion is, with or without his hands.
Laura on the other hand has just gone on a complete journey of self discovery and learning how to dream again. A journey to center herself, find where she belongs, to find a way to love herself, to forgive herself, and to accept herself. During that journey she did care for Hellion. He held a special place in her heart. He pushed though, his impatience dictated too much, he became needy, clingy, and wouldn't listen to her. He showed signs of disrespect though Laura was cutting him slack at every turn as she's grown to be an empathic and understanding young woman. There was a breaking point where enough was enough. That dawning realization that he's just not good for her. That if they stayed together, he would have brought her back to who she was. That person that she's grown out of. A person she refuses to be again. His insecurities infected him to the point it would have consumed everything he cared about. He's just not at that emotional maturity level to handle it yet.
Life is like that. It's dominos. One action will ripple through everything. It can ruin everything you've tried so hard to build in a relationship and continue wrecking everything in its wake including friendships. Laura won't be looking back to Hellion as he is now, she'll push forward to continue bettering herself and making herself whole. Sadly, Hellion won't be. He'll be forever lost in his mind of what did he do, or calling her names to make himself feel better, or staring forever on into the woman that won his heart that he pushed away like he did everything else because of his anger and misplaced need to feel superior.
What Laura did wasn't cruel. The little boy that Hellion always has been is. She'll move on to bigger things in life, while he continues to throw tantrums about what the world has taken from him. She's out there doing something about her predicament, and growing from it. He's not. Not yet at least. It'll take years for him to come to terms with himself and finally grow, but that's real world time. In Marvel time it may be decades.