Beyond the folksy humor and great coffee, there is an impenetrable layer to Twin Peaks. The Lynchian nightmare that is the Black Lodge has bled into the rest of the world like weaponized metaphor. It is a world of unexplained incidents and paranormal made routine. There is no greater example of this than in the long-debated topic of BOB—the creature behind the man, the thing that killed Laura Palmer, and the evil entity controlling (one version of) Kyle MacLachlan's Dale Cooper in season 3. What David Lynch presents us with is a puzzle with only a few of the pieces to work with. Whatever truly is we’ll probably never know for certain, but the pieces we have do hint at the larger whole, and asks the question of the nature of evil.
As established, BOB is an entity rather than a person. What kind of entity is uncertain. He’s lived in some capacity in the woods surrounding Twin Peaks, near the Black Lodge, where Leland Palmer first encountered him as a little boy. This lends credence to the idea that he is a woodland spirit - at least in origin. Instead of food, BOB feeds on garmonbozia (fear and suffering) that takes the form of creamed corn. Of course, this makes little sense to anyone who has ever had cottage cheese or greek yogurt, but that is neither here nor there. In this way, BOB is not just an entity, but a parasite as well.
As BOB, Leland victimized his daughter from the age of nine before murdering her at eighteen. The reason BOB/Leland got away with it for so long was because of the entity’s abilities. When BOB was in control, Leland was unaware, and others—like Laura Palmer herself—only saw BOB, not Leland. Laura found out the truth on accident, and it set off her final downward spiral.
BOB isn’t a demon or spirit in the traditional sense. Following Leland Palmer’s suicide, Agent Dale Cooper speculated that perhaps BOB was the personification of the evil man was capable of committing. Not only parasitic, BOB is also a chameleon. Or, rather, he’s a catalyst whose actions reflect that of the person he’s possessing.
For instance, look at Leland Palmer. In Fire Walk With Me, BOB/Leland was orderly, and measured in his acts. He attacked Laura when he knew he could get away with it, and acted only when he knew he wouldn’t be discovered. This is reflective of Leland, who, prior to Laura's death, was a measured and collected man. He went about cheating on his wife with prostitutes and dealing with degenerate criminals, thugs and pimps with an ease of action and personality. Whether BOB influenced these illegal actions or if this was entirely Leland acting on his own to BOB’s joy is unclear (oh, Twin Peaks).
However, when confronted with the realization that Laura was one of the prostitutes he was visiting, he reacted with terror. His actions were sudden, jarring, and bizarre. He acted the same when confronted by Mike. This is likely for two reasons: that BOB fears being discovered, and that Leland at an instinctual level is becoming aware of what’s happening to him. BOB’s next actions were then more overt, acting suspiciously in front of Leland’s wife.
After Laura’s death, Leland was erratic and manic. He had poor impulse control and engaged in self-destructive, overt acts, like murdering Jacques Renault. In return, BOB murdered Maddy Ferguson and attempted to kill Cooper with a nine iron - the latter done in broad daylight and nearly guaranteeing his capture if successful. The capricious nature of the attacks suggest a sudden change for the erratic, similar to Leland’s own personality at the time. BOB’s actions always seem to reflect Leland’s mindset. One feeds off the other symbiotically.
Now, look at the actions of BOB after taking control of Dale Cooper; the crimes and MO are different. In Leland, BOB freed a sexual violence that existed in the depths of Leland’s soul that Leland himself may not have been aware of (this is depending if he was actively cheating on his wife; if it was a desire he knew he had, BOB simply lit the spark).
BOB/Cooper is different. In the revival, he has the organizational skills of Cooper, but not the sexual sadism. His crimes are to the service of his overall goals. It’s focused, manipulative and planned. As a character, Dale Cooper is morally righteous. Villainy offends him dearly, but he isn’t squeaky clean either. He easily manipulates suspects and slept with his former partner’s wife. In his investigation of Laura Palmer’s death, his plans were well-conceived and prepared with every eventuality in mind. BOB/Cooper catches Darya by bugging her phone and interrogating her using circular questioning, similar to the way an FBI agent would catch a crook. He killed Garland Briggs and made it look like an accident. In South Dakota, he instigated a closed-circle crime so that everyone involved ended up in jail and everyone who knew his face was dead.
BOB even created another copy of Dale so that even if the real Dale managed (after decades!) to escape the Black Lodge, BOB wouldn’t be forced back into the para-dimension—the new clone would. Contingencies for every eventuality. BOB scammed Gordon Cole and Albert Rosenfield by playing to their friendship—that thumbs-up was a step too far!—the same way Cooper once scammed Bobby Briggs and James Hurley by playing them off each other in order to find out if either of them killed Laura.
These aren’t the actions of an entity that has an MO that can be programmed, categorized or easily referenced. Like Hannibal Lecter, BOB wears a metaphorical person-suit and engages in the actions that the person he possesses would do themselves if they were as base as BOB himself. BOB obtains his garmonbozia not only from the lives he takes but from the person he controls having to witness their deepest, darkest possibilities laid out for all to see. BOB isn’t a demon—he’s the enemy within.