If Cotton was a violent, abusive guy--the type who would terrorize his daughters--then any challenge to his authority could have set him off. His personality--or what was likely narcissistic personality disorder--precludes the need for any specific motive.
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TT has been described by FW_Cat as crazy and empty. Is there evidence of his being physically violent (domestic abuse allegations, assault charges, etc.)? Did he has have a short fuse back in the day? The way he's been described makes him sound somewhat aloof. Judging by other cases I've followed (and assuming the culprit wasn't a stranger), the type of person who would commit this crime is the type who would be very volatile--prone to sudden, violent mood swings.