There's a theory going around that Walt keeps mental or physical trophies of the people he's killed
-Cuts the crusts off his sandwiches after killing Krazy 8
-kept the eye of the teddy bear after he indirectly killed the passangers on the plane crash and Jesse's girl
-keeps the book Gale gave him
-maintains a mannerism similar to Gus
-pours his drink on the rocks after Mike's death
the disturbing part about all this is.... watch the opening of the first episode of this season.... the eggs and bacon....
"You're blowing my mind, Frank"