And she doesn't get killed off, she's just left out. Omitted, Not mentioned. Not actually killed. Relax.
I forgot pretty much the whole movie except Del Torro shooting an entire family in the head during dinner...I think.
You got it wrong: it was mobster Del Toro whose entire family was killed at the end during dinner...I think.
If she had gotten killed off at the beginning of "The Girl On The Train", a whole lot of misunderstandings could have been avoided in THAT film.