Well, based on my understanding of it, those "lostbelt" things don't have any future, right? They're doomed. [If she didn't "stop" them, they would end up as dead worlds regardless before long. That was what she understood at least.] So I don't think you should feel bad for erasing them when their existence threaten the reality that has a future. Besides, it's not like you're not doing it just for yourself, either.

[ She closed her eyes and looked down for a moment. ]

When I was trapped in the game, I... Did the things I did for my own sake only. [She took a deep breath.] Nothing was real around me, so I started thinking even my friends were not real. No... I convinced myself they were not real. That way I could be selfish and not have to deal with guilt. [And even then, evidently enough that hadn't worked.] I think that's what makes a heroine. Selflessness.