Board Thread:General Discussion/@comment-28032065-20160817092833/@comment-29690476-20160820070214

According to how the Genocide route ends, it's possible that Chara was trapped underground, blocked by the barrier(with all monsters killed, the requirement to pass the barrier can't be reached). They are also aware to this fact, so they recreate this world in order to let the player find a way to leave underground. We may assume that the only means to leave is to achieve the Pacifist ending. Then Chara was able to get to the surface and complete their plan.