This is why arachnophobia is not healthy. Spiders are everywhere, and an irrational fear of something found everywhere is going to impair one's behavior to a severe degree.
Nah, the twist is that the spider was in on it. It was an insurance scam to murder her child and collect the insurance money. In return, the spider would get 15% of the cash.
This is like a real life "cut the red wire or the blue wire" situation.
phobias aren't exactly logical, what she did was totally instinctive...you can have irrational fear of anything. not defending her but it's not like she was thinking about it, she panicked.