The statement that "intelligence borders on insanity" seems to describe DP perfectly. He has a great criminal mind, but no matter how intelligent a person thinks they are there is always someone a little bit smarter that will bring them down!
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It is a proven fact that high IQ'S border on the edge of "insanity". What makes a person's mind cross over to insanity is unknown. Maybe, it's that first full rage, like DP had with Kathy, and then it just lurks behind the "calm" until something triggers it again. In this case, it was Stacy saying "I'm leaving".
IMO, this is far more dangerous in the mind of a "police officer" who has been trained to look for clues in murders, etc., and then when they commit one, they have all the prerequisites to "think" they will get away with it.
IMO again, DP could have very easily gotten away if it wasn't for Cassandra's call that night. Since then, his mind has to be "breaking down" and thus his lies come out.