It really shouldn't be in-character for Illyana at all. If Louise Simonson had remembered her origin after she took over from Claremont no writer would ever depict her as quick to kill. But unfortunately, because Simonson couldn't be bothered to reread the Magik limited series and see that Illyana spared Belasco because of what she feared killing him in anger would do to her soul, later writers have gone with her 'I can kill with little-to-no consequence' interpretation.
Simonson legitimately thought this moment ended with Illyana killing Belasco, which is why she had no problem killing people in Fall of the Mutants and trying to kill Forge, and probably why N'Astirh was created in the first place. Since Simonson thought Belasco was dead a new demon needed to be created to take his place in Illyana's final story.
If they were smart they'd say that she only kills when she deems it absolutely necessary and never out of anger, and that if she ever killed while truly angry she'd lose herself and go full Darkchild, kind of like how Jedi can kill, but if they do so in anger they risk falling to the Dark Side.
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