Have you seen what she went though? It's obviously mental illness.
She literally
wanted to leave and was about to do just that, but then for some reason this one person being mad at her, despite the JSDF (and the guy SHE WANTS TO BE WITH, can't emphasise that enough) defending her, she decides that her place is dying here and having no future. That's not mental illness, that's the author wanted there to be drama but couldn't be arsed to come up with a reason. She goes from caring about life and wanting to be with the guy she loves to being suicidal because some rando yelled at her.
It'd be like if when they rescued Noriko and told her they'd get her back home to Japan, if along the way somebody said a mean thing to her and because of that, she decided "Actually no, don't save me, I'm going to remain here and be killed".
What's even crazier is they just leave her there to her fate. Like wtf? Take her with you! Let her hate you later, just save her life and at least
try to convince her her life has value!