When Neil comes home after losing Waingro, in the beginning of the movie, we have this famous shot.
He comes home late at night, drops his gun and his keys on a table, then go stand at the window to stare at the Pacific.
The shot is heavily inspired by Alex Colville's 1967 painting, Pacific, picturing a male figure watching the ocean from an open bay window, with in the foreground a table with a gun on it. We can see in the painting that same loneliness that exudes from the shot.
This painting is commonly felt to be about suicide. We can link that to Neil's literal death at the end, and/or to the slow suicide that the 30-seconds credo has turned his life into (turning him into a ghost, another thing that has been said about Neil in this shot).
The gun on the table is Colville's, that he was issued when he did World War 2. This is also a loaded point. In the suicide interpretation, this would mean the man is about to kill himself with the violent tools given to him by the state. Similarily what has turned Neil into a ghost is the violence that was given to him by the state - the ability to fight to survive in prison, the ability to withstand abandonment and loneliness, which he uses to follow his credo. The ability to use guns, learnt in the army, which he uses to rob, which forces him to be alone, etc. A state-given capacity of destruction.
The table is also significant; it is a sewing table that belonged to Colville's mother. If the gun is destruction, the sewing table is creation.
There is also a possible interpretation saying that the painting isn't about suicide at all. The man would instead be lost in thought, turned away from the world, maybe unable to choose between creation and destruction. This can be linked to Neil's choice at the end: leave with Eady, or kill Waingro?
It can also be more complex. Maybe it is not about picking one between creation and destruction, and instead about finding balance between the two. After all both are needed. This search for meaning is supported by the heavy influence of French Existentialism on Colville, for example Sartre and Camus (let's note that Camus is on Neil's reading list in Heat 2), that tried finding meaning in the face of nihilism and unthinkable horrors such as WW2. (source)
Finally, in the movie the foreground elements (visible on another shot) are his gun, his keys, and the table. If in the painting the two important elements are the gun and the table, in the movie it would seem to be more the gun and the keys (the table doesn't have any particular value to Neil). The gun being destruction, and the keys, maybe instead of creation, would be belonging.
In short; Neil, reeking of self-imposed death, unable to find meaning and torn between his capacity for destruction and his yearning for belonging?