[[WMG: Becoming the Iris Hunter was necessary for Kuga's survival.]]
Hijiri saw a death butterfly on Kuga from the moment Houjou made his false love confession to her. However, the events that sparked Kuga's Iris hunting seemed to come from overhearing Toru and the others' confrontation with Houjou, which made her doubt her boyfriend enough to use her Iris on him and see the truth - the most recent in a long string of unhappy revelations from having her Iris, drawing her to the conclusion that things would be better without them. Yet she was already on the way to her death even before Toru's early efforts altered her fate.
If that confrontation between Houjou and the others had not happened, or she had not overheard, then Kuga would have gone on allowing herself to ignore her Iris and the relationship would have persisted much longer. Long enough that, when it finally did come apart, it would be far more traumatic than just her crush stringing her along for a week or so, and in a way that would make her burden herself with guilt and self-loathing rather than blame her Iris. Badly enough to make her commit suicide, even - and in a place or situation where nobody else would be able to witness or prevent it.