Shawna could have done better

spoilers
I just dont get it. If someone put a thin plastic bag like that over me, my hands wouldnt be at my neck,I would poke a hole or tried to rip the bag. Also, if I would have walked in to my best friend on the floor and I found out my friend's nanny was a psycho, I would have called 911 right then and there.
It was an okay movie, could have been better.