Continuity mistake: During the van chase sequence, the van door is open for the first few minutes. Then the van makes a sharp right turn and immediately it cuts to the door which is now closed. In the immediately following shot, it's back open yet again.
Continuity mistake: As Tracy Atwood is catapulted from the open van door onto the taxi's windshield, the side view of the taxi shows the side windows have a curtain or black film behind the glass. Immediately after, the next side view shows no curtain or film, and the driver is now visible. Slo-mo is not necessary.
Continuity mistake: During the scene when Tracy Atwood is catapulted from the van, it was because the rear part of the van slid into another vehicle. As the van drove away there was no visible damage to the passenger side of the vehicle.
Chosen answer: It sounds like Dissociative Identity Disorder. And I think compulsive killing is something he chooses to do, and not connected to his DID; and from skimming the article, it doesn't appear to be genetic.
Cubs Fan ★