Before exploring any questions as to how the phenomenon of sadism might apply to fiction generally or the comic-book medium specifically (see here), it's necessary to establish a theoretical judgment of the phenomenon's nature (as opposed to a judgment on what if anything could/should be done about it).
Most educated persons know that for however long the actual phenomenon has been around (I hold with those that think it goes back to prehistory), the origin of the word "sadism" was coined by sexologist Kraft-Ebing in his 1890 work PSYCHOPATHIA SEXUALIS. In this book Kraft-Ebing derived his term for a subject's taking pleasure in others' pain or discomfort from the name of the Marquise de Sade: thus"sadism." Kraft-Ebing also took a similar literary model for the phenomenon of taking pleasure in one's own pain: "masochism," named after Leopold von Sacher-Masoch. I'll be touching on both phenomena here though I'll focus on the former since I don't believe any significant voice since the days of Frederic Wertham has suggested that popular fiction might turn readers masochistic.
Sade's books, starting with JUSTINE in 1791 and published to increasing scandal until (and after) Sade's death in 1814, had only one purpose: to lovingly describe harrowly-explicit scenarios of murder and torture visited upon innocent victims (mostly young women) by ruthless aristocratic voyeurs. According to critic Mario Praz, the "Divine Marquise" had a titanic effect on the development of European literature from then on, to say nothing of creating an entire erotic subgenre that developed in Victorian pornography. Not unlike a lot of literature deemed too "popular" in nature, Sade's works possess an ambivalent status in literary studies, at the very least maintaining a certain cachet for historical reasons.
Naturally "father of psychology" Sigmund Freud had his say on the sexological terms introduced by Kraft-Ebing, and he attempted to define sadism in a causative continuum with masochism. Though many of his precise theories changed during his career, his basic assertion was that sadism was the more primary phenomenon. It arose from a subject's aggressive, even "phallic" desire to inflict pain on others for his own pleasure. Given Freud's general emphasis on guilt and repression, it's not surprising that he defined masochism as pleasure arising from a subject who feels the need to inflict pain but who disavows the impulse and turns it upon himself-- sort of a "self-castration" complex.
Philosopher Gilles Deleuze overturned Freud's formula, using what he called a "literary approach" to disassociate masochism from sadism. He did so by examining in depth the concepts behind the respective syndromes as put forth by the fictional works of Sade and Sacher-Masoch. In addition to finding that the two syndromes had radically-opposed etiologies than those asserted by Freud, Deleuze found that masochism seemed to be the more primary phenomenon, arising during the subject's pre-phallic phase. As Gaylyn Studlar put it, "Deleuze considers masochism to be a phenomenology of experience that reaches far beyond the limited definition of a perverse sexuality" (IN THE REALM OF PLEASURE, p. 14). However, Deleuze agreed with Freud that, even if the etiology of the masochist was not covalent with that of the sadist, the masochist disavowed "phallic power" by suspending his own egoistic desires, thus making it possible to interpret his pains/discomforts as pleasures. The masochist wants to feel overwhelmed by a dominating force-- although he always contracts in advance with the "force-giver" as to what type of force it will be-- while the sadist wants to be that force toward others who categorically do NOT interpret pain as any kind of pleasure.
Though I agree with Deleuze in his distinctions between sadism and masochism, I think that both Freud and Deleuze are guilty of over-intellectualizing the somatic aspects of these sexual syndromes. "Disavowal" is just another intellectual construct devised to emphasize "absence" rather than "presence," thus putting both thinkers in line with similar types like Sartre and Lacan. I would emphasize more the aspect of bodies clashing against bodies, which IMO is the main reason that either activity summons up associations of sexual excitement. With this caveat in mind one can schematize the respective attitudes so: the pure sadist wants to actively inflict his power/strength upon others without opposition; the pure masochist wants to have the power/strength of others inflicted upon him, albeit under controlled conditions. I prefer the term "strength" to the now-dated term "phallic power" employed by Freud and Deleuze, since the former term does not limit itself to the phallically-endowed gender.
And this leads me to consider sthenolagnia.
Many sexual syndromes concerns activities not common to what we consider "normative" (in terms of statistical concentration) sexual stimulations-- crushing bugs, odors, or numerous other triggers. The sthenolagnia syndrome, however, bears some marked resemblance to normative stimulations, for this syndrome deals with stimulation by strength or musculature. Obviously, as per my earlier example of women's romance covers, at least one gender is stimulated enough by male musculature that such books sell in dependably-high numbers. In contrast, normative male stimulation doesn't focus upon strength in the female gender, and if anything femininity is defined in some minds as the absence of pronounced strength and/or musculature. However, even within normative, non-fetishistic sexuality, men have been known to fancy women with strong legs that suggest superior lovemaking skills (Crumb again!) and women have been known to seek out less imposing men, possibly in part to satisfy a nurturing instinct. Or perhaps both of these are pat explanations, much like those of Freud and Deleuze, and it comes down to the possibility that humans just like more variety than other animals.
I should note also that Freud's emphasis upon pain as the constant element to both sadism and masochism has less application to bondage that my theory of strength (stheno= forceful) as the common element, for as Marston pointed out, in bondage the captive is subjected to force but not necessarily to pain. I have no idea whether or not tenured sexologists could ever find a definite link (insofar as any of these heuristic speculations can be definite) between sadism/masochism and sthenolagnia in their researches. But like Deleuze, my main concern is the apperance of these syndromes/fetishes in literature, to which I'll be turning next.