[quote=Biscuits]
Even then, it didn't feel like there was any trigger for it. Even a psychopath has their reasons, but it just felt a bit like caelums one got selected out of the blue
[/quote]

SHe called her by a nickname she didn't like.

noun, plural psychopathies. Psychiatry.
1.
a mental disorder in which an individual manifests amoral and antisocial behavior, lack of ability to love or establish meaningful personal relationships, extreme egocentricity, failure to learn from experience, etc.

I'd say that Nicki is experiencing several of these - Antisocial behavior, extremely egocentric... I can find more, if you want.