When an affair happens to get between two people; the husband being obviously in love with his wife; it's easier for Eric to blame Sean than his own wife. It's common in relationship when one is caught cheating on the other. It's as if the pain is too much for the one cheated on to grasp, so automatically the blame is projected onto the one the spouse/etc is having the affair with. When in reality, the spouse is guilty. If only Eric had enough self control to just call the police, not go and pick up that gun.. he could have divorced her, and no doubt would most likely have been able to get full custody of their two children, could have started his life anew. It's hindsight and in the heat of passion (i would say it was a killing out of his passion for his wife) he couldn't think clearly and was impulsive. You can see it in his eyes in the pic on the first page, first post's link. It seems as if he shot Sean in a blind fury. The minute he knew of the affair to begin with, he should have filed for divorce. I don't see how trust could be gained back after such an incident.
Erin is clearly unstable, i'm wondering if family out of state is covering for her and her kids. Otherwise, you would think the hunt for them would be stronger due to her mental state.
