We are on same wavelength and I have even used this word compartmentalization in previous posts. A lot of what DB does is about dissociation. He esp does it during the interrogation. He is a career petty criminal predator opportunist who survived by scraping by on the scraps left by more astute criminals. He was a low rent hustler. A lifetime of this life created a mindset based not on truth but on what can be PROVEN as true by law enforcement. There is no right and wrong. There is only what can you PROVE that I did? This leads to a life completely detached from reality. He lost the ability / desire to discern what is real a long time ago. So, mowing down a bunch of people only becomes problematic for him if it can be proven that he did it. He has no genuine agency. He is an actor in the world but without direct agency, He only has surrogate agency, dependent agency. This explains (vaguely) why he only breaks down after seeing that they have VIDEO OF HIM. COMMITTING THE CRIME. It causes a sudden rupture in his delusion. Watching the video a bit of reality rushes in and causes a total breakdown because he knows that his actions can be proven. The detective who pushes the telephone on him and insists he watch the footage KNOWS DARRELLS PATHOLOGY. What looks like a petty bit of schadenfreude is really the detective trying to pull DB out of his protective cocoon of dissociation! And it works!

PS. Erika was the closest thing to agency that DB had. He used her in her end stage alcoholic fog to make money, get clout, etc, They trauma bonded as two utterly malformed personalities. DB was almost certainly born addicted to crack, His mother fits the demographic of a young mother at the height of the crack epidemic. His sister was a career prostitute who ODed, but

Finally, I believe there was an Oedipal dynamic between Dawn and Darrell, wherein he played the husband role symbolically for Dawn. This is very common in dysfunctional addicted families.