In the beginning of issue 3 we see the first instance of Illyana's suicidal tendencies.
running.jpg
In issue 2 Illyana fought to live because she still had hope she could go home again and everything could be as it was, but as she comes into a fuller understanding of the struggle between light and dark raging within her, and as the passing years reduce her hope of ever seeing home again, Illyana has come to see death as means of escape from her torment. She would let herself be killed here except that she knew Belasco would just bring her back, a prediction which turned out to be entirely correct. While the first 3 years in Limbo were a struggle to stay alive, the last 4 years were not because Belasco would not let her die. With that easy way out denied to her, she had no choice but to go through Belasco.
This desire for death is one of Illyana's more consistent character traits. After everything that happened in Limbo she hates herself with a passion and her greatest fear is losing control of her dark side and killing or corrupting everyone she cares for. In the issue where she joins the New Mutants she wishes she had not survived Limbo. In the New Mutants special edition in Asgard the Enchantress says she will grant Illyana the 'oblivion' she seeks. When the Beyonder comes to kill the New Mutants she immediately offers herself up as a sacrifice to spare the rest of the team. In issue 50 she tries and fails to kill herself while fighting S'ym and the tech-virus infected armies of Limbo. She states 'I should be dead' during Inferno and this time she succeeds in committing suicide, leaving an alternate timeline version of herself behind to take her place. When Belasco brings her back, she is furious not only that he brought her back without her soul, but that he wouldn't let her stay dead. During Second Coming: Hellbound she begs Pixie to kill her, seeing herself as no better than Belasco. In Dead Souls when she thinks Rictor died during a mission she asks why her story keeps going, and when Rahne died in Rosenberg's run she again expressed her frustration that a friend died instead of her. Before Krakoa she saw death as the way to end her suffering and protect others from the evil inside her.
This is why the Truth or Death mini is so out of character for Illyana and is best taken as non-canon. The 616 Illyana both never expected to survive into adulthood and didn't want to. Coming into the future and learning she was no longer alive would be as surprising and upsetting to her as learning that water is wet. The question was never 'why did she die.' It's 'why did she live as long as she did?' It's why she didn't commit suicide before Inferno, why she didn't let S'ym or the demon bear kill her like she did the Beyonder. It's why she got up in the morning and helped defeat the Shadow King or Kulan Gath or Legion, why she kept going when living was so hard. Why did she wait so long to make the choice Jean made so quickly after becoming Dark Phoenix when she wanted do die far more than Jean did?