In Season 1, Don's domineering nature, and suspected cheating, lead to his wife developing a nervous disorder. Unsurprisingly, he denies her the help of psychological therapy, but eventually relents when the symptoms worsen. As if this wasn't bad enough, he uses his relationship with Betty's psychologist to spy on her, getting him to pass the notes from her sessions, and report back on anything she says which might be of interest. Not the best way to build a marriage. Ultimately Betty finds out, and discontinues her therapy sessions. Trapping his wife in a web of manipulation like that is especially bad when you consider he was responsible for her breakdown in the first place. Ranking these sins is difficult, but while some of his crimes could not have been predicted and are indirect results of his actions (the suicide of Lane Pryce for example), this is a particularly sneaky one.
