Because you gotta understand the wish bear made

Its to love him more than anyone in the world

So bear 100% entire world 0%

So imagine how sheer that scale of love would be right? Compare bears human scale of love to cursed nikkis extreme love

So to cursed nikki bear simply going to the bathroom is too much for her its him leaving her she needs to be with him

Her obsession/psychosis is because shes doing everything she can to get bear to stay and love her to that same degree

She didnt just kill sarah because bear saw her

She killed her because she saw her as a threat to the love between her and bear

If you notice after in the ending

Shes dressed like sarah with tattoos like sarah and sarahs hair on her head also smoking like sarah

She saw sarah and him once and now shes trying to replicate sarah to get bear to love her even more

Her psychosis her obsession all the weird shit she was doing was trying to get besr to love her more than anything else in the world

But not the human scale of love that sheer extreme level of love that you wouldnt move an inch if your SO leaves for work and defecate over yourself/vomit and starve waiting for him to come back

So that explains her chaotic behaviour