Of course, she should have taken Joseph with her, she should have snatched him from the buggy, or run with the buggy (Jessica would have). It didn't matter that Joanna was only six years old, that she would never have managed running with the buggy and that the man would have caught her in seconds, that wasn't the point. It would have been better to have tried to save the baby and been killed than not trying and living.
Joanna has survivor's guilt. She is also looking at a situation that she endured as a six year old through the eyes of an adult. Of course, as an adult, she feels that she did not do enough, and that she should have snatched up the baby, stood up to the man, made an attempt to save her family. But as that child, she was not thinking with the mind of the adult she would become. Therein lies the confusion of Joanna's guilt; she is thinking of the events looking back as an adult version of her six year old self, requiring adult behavior from a child still in elementary school, which is what makes everything almost impossible to come to terms with.
Andrew Decker didn't destroy his own family, he destroyed someone else's. He destroyed Howard Mason's. Men like Decker were inadequates, they were loners, maybe they just couldn't stand to see people enjoying the lives they never had.
Louse is is analyzing Andrew Decker's motivation. Howard Mason's family had done nothing to him personally. This was not some retribution killing, however misguided. This was killing out of an exaggerated sense of jealousy. Decker has the mentality of someone who vandalizes the property of another person because they are jealous that they own it; the person who sees a Maserati in a parking lot and deliberately scratches it, or the jealous classmate who deliberately ruins the fancy new outfit of another they are envious of. Decker's envy is insane in its extremity but it is out of anger at his own life, and not anger at the Mason family's, that he kills. He does not have a loving family, the love of a wife, nor has he ever felt the deep bond with his mother that the Mason children clearly have. He wants to destroy it when he sees it in others, because he believes this will make the emptiness that he feels easier to bear.
