I watched it with a mother's eyes and heart and that's what stood out for me in this movie.
In my viewing experience, BirdBox is NOT about the killing and self-killing, it's not about the creatures and the supernatural. It's about personal growth and a challenging journey to motherhood.
The acting performances are amazing, even though sometimes a bit too predictable (John Malkovich is basically the same character in all his movies!). The banter between Sandra Bullock's character Malorie and Malkovitch's Douglas was a nice touch though. And I have been particularly impressed with the little girl.
(SPOILER AHEAD!)
When Malorie and the children (Boy and Girl) are approaching the rapids and Malorie said one had to look, my heart dropped when the little girl volunteered when I could feel that Malorie was considering her to be the lookout, maybe because she was not her biological child. You could see the dilemma in Sandra Bullock's face and all the bravery in that simple phrase that Girl dropped "I'll do it." Interestingly, Boy had volunteered first but had been cut off by Malorie who said she'd decide. So I think the performances of both Sandra Bullock and the little girl really made that scene powerful.
I'd read the book now thanks to the movie!