Some further thoughts on how the Deep Purple song is being used in Saul:
“Smoke on the Water” is first heard being hummed by Marco in the opening flashback of 1.5, as he pretends, as part of a con, to be a near-unconscious drunk in an alley. It appears to be associated with his belligerent attitude toward apparently all comers, the “buttholes” he wishes to “roundhouse kick.” Since, apparently unlike Jimmy, who consistently selects wicked or extremely shallow marks, Marco does not seem to care who they scam - note his cheerful assuming that Jimmy must be fleecing his elderly law clients - this speech suggests the element of conning he is addicted to is aggressive dominance. (Since time is short on television, characters’ lies tend to be unwitting revelations of deeper truths about them.)
Jimmy later hums the tune as he drives off in his car at the very end of S1, after insisting to Mike that he will never be held back from a financial opportunity by “the right thing” again. He has just returned from Cicero following Marco’s death and while wearing his mafioso-style pinky ring, indicating that Marco’s attitude may have been contagious.
As the earliest chronological reference to the name “Saul Goodman” is in the same flashback scene, which is also the first time we see “Slippin’ Jimmy” at work, there is reason to link the guitar refrain to both aliases. While Slippin’ Jimmy might suggest a loss of control, hence absence of malicious intent, Saul Goodman brings to mind a thoughtless, even devil-may-care attitude - a vocal rejection of guilt enabling even guiltier behavior.
In the Deep Purple song, the refrain seems to represent both the casino-destroying fire as well as the persisting pall of smoke that reminds the band that such destruction could happen again someday. It should be remembered that the fire causing Chuck’s death is not so much foreshadowed as directly prophesied, and more than once, on the show, with his doctor and the police both warning that the indoor gas lamps are terrible fire hazards, especially given his many stacks of loose paper. Death by fire is thus offered to us as the likely tragic endpoint of his disease, meaning, since it is made clear on multiple occasions that Chuck’s symptoms are closely bound up with Jimmy’s “slips” into illegal or professionally unethical practices, that the belief that Jimmy has slipped all the way downhill is likely to be the harbinger of that end.
It is Chuck himself who kicks his lamp onto his papers, however, even if his final relapse has been presented as the result of the sabotage by Jimmy of his insurance which had ended his legal career. Chuck’s lamps are presented in seasons 3 and 4 as the knowledge he imparts to his younger brother to guide him along the right path in life, perhaps signifying that it is the combination of this leader role with the law papers (the letter of the law that Chuck adheres to at such great cost?) that is the real trouble. Jimmy’s descents into a Marco-like attitude of vengeful callousness, the most dramatic examples of which seem to arrive toward the end of each season, are never self-caused, after all, but come about in reaction to revelations that Chuck (and later his stand-in, Kim) don’t believe he can ever be a true lawyer. It should be noted, too, that since Chuck’s condition is a delusion the wickedness he perceives in his brother’s rule-breaking propensity may be as well. In short, even though Jimmy, in his Saul phase, can be identified as the fire that will burn up Chuck, it may be Chuck himself who is responsible for the setting of that fire.
Where characters are concerned about Jimmy’s corner-cutting (or outright crimes) they tend to use figures of speech involving flames, e.g. Kim’s telling him he’s “playing with fire” by giving fabricated clown fetish videos to the police. The color consistently associated with Jimmy’s and others’ rule- or law-breaking behaviors on the show is red, which is of course a “hot” one, and in his slippier phases Jimmy is also shown preferring tropical drinks and spicy foods. When acting within the law, though, Jimmy’s short cuts are more along the lines of clever but permissible slashings of (ironically) red tape, enabled by his remaining focused on his clients and whatever pertains to their well-being rather than statutes, documents, and protocols the way Chuck would.