Apart from the general malevolent scheming, Bruce also seeks to satisfy his cravings for violence, drugs, sex, and pornography along the way while happily voicing his racism, anti-Catholic sectarianism, and misogyny, all the while pining for his ex-wife.

As the novel progresses, Bruce's mental health begins to deteriorate; it is revealed that he suffers from drug addiction and bipolar disorder, which, along with his inability to form meaningful, trust-based relationships, are exacerbated by latent, unresolved psychological problems caused by childhood abuses.

Eventually, Bruce is forced into taking leave due to injuries he suffers while dressed as his ex-wife, leading to the revelation that he committed the racially motivated murder that serves as the novel's main plot and that the colleagues he despises – particularly his boss Robert Toal – have been aware of his guilt all along and have been protecting him from the consequences of his actions out of a mixture of loyalty and pity.

Additionally, the tapeworm – already distraught over the loss of the other (as a result of medication Bruce's doctor prescribed) – is expelled with his waste and dies with the host.

Also, Bruce uses rhyming slang, a common element of urban Scots dialects (such as "Sherman tank" – wank; Demi Moore – "hoor" (i.e. whore), Jackie Trent – "bent", and so on).

At a certain point in the book, the narrative starts to be interrupted by a tube-like structure that appears on top of the text, and at first is only made of the word "eat" being repeated amid the zeros that fill the empty space within the tube.

In Filth Bruce kills Gorman by biting out his tongue, causing him to reel backwards and fall out of a two-story window, where he suffers fatal head injuries.