This general idea about investigating new kinds of crime is one of the starting points of Mindhunter but it's not strictly correct. It's eye-catching and topical for the late 1970s, with rising concerns about mass media, and in the context of the show itself it might be a little bit of a tweak on a 2018 viewer having reached peak true crime media saturation post-CSI, post-Investigation Discovery network, etc. But this thesis defining the modern criminal as the agent of an entirely new breed of "senseless violence" and linking that to TV consumption is much too broad. The reality is really messy, and the show spends a while exploring those realities even if it isn't saying that in as many words.

There's some difficulty projecting late 20th century categories and methodologies like the "serial killer" into the past, but people who killed for seemingly-inscrutable psychological reasons, out of sexual sadism or internal compulsion, did exist and were recognized as such prior to the advent of television. Either you have to file senseless cruelty for the gratification of its perpetrator alongside more emotionally-intelligible motives like crimes of passion or crimes of greed, or these kinds of crimes aren't necessarily new. Even restricting ourselves to relatively well-documented 19th and 20th century examples: the child serial killer Jesse Pomeroy who killed between 1871-1874, French serial killer Joseph Vacher active in the 1880s and 1890s, or the unidentified perpetrator of the Austin servant girl murders of the 1880s, or German serial killers of the 1920s and 1930s like Fritz Haarmann or Peter Kurten -- all of these might be described in terms of murder and mutilation beyond what was necessary to obtain a specific concrete objective, even the concrete objective of rape. Even a category for such killers was not entirely unprecedented. The earlier category of Lustmord, the lust-killing, was articulated by 19th century psychologists as an explanatory model for sexually sadistic homicides and their perpetrators.

A decade earlier the same character might have been attributing this "new" breed of criminals to the interstate highway system, or to the domestic effects of World War II and women entering the workplace. A decade or two later, he might attribute this new breed of criminals to the alienating effects of modern urban planning, or post-Vietnam War disaffection, or the ubiquity of child abuse in the first 3/4ths of the 20th century, or the effects of leaded gasoline and lead paint. These competing theses for serial murder as a phenomenon all have merits, but not a single one of them is comprehensive.

The rise of television certainly does coincide with the rise of the serial killer as we know it in the 20th century, if you consider "the rise of television" and its emergence into ubiquitousness to date to the 1950s -- replacing radio and even diminishing the newspaper, both of which featured not-insignificant depictions of violence. The data sample and the interviewees that the real-life team of Douglas, Ressler, and Burgess were working with (the real-life equivalents of the show's Ford, Tench, and Carr) were skewed toward notable offenders born post-1940 or so -- adult criminals who had been active as sex criminals and murderers and subsequently had been apprehended. The demographics of these offenders are shaped by which offenders Douglas, Ressler, and Burgess had access to -- they had to be willing to comply with their interviews, they had to have an accessible case history for comparison purposes, they had to be geographically accessible and to have not been executed, and so on. This resulted in a pool of relatively recent offenders. They weren't digging back into the past for a comprehensive view of all prior criminals and their patterns; they were seeking to understand a phenomenon that appeared more striking and apparent due to the relative accessibility of such offenders in an increasingly connected and organized era of law enforcement and criminal justice.

This specialized law enforcement focus coincides with an uptick in inter-departmental and inter-state communication, several high-profile serial killer trials and films about serial killers, and technological advances (like fax/teletype technology). All of this took place post-1950 and in some cases within a relatively brief period of the 1970s, despite being in the works since the 1800s. You might compare the effects of these factors on the modern understanding of the "serial killer" as a figure to Ann Burgess' own work and that of her peers regarding sexual assault trauma and child molestation -- child molestation and "sex perverts" were a subject of cultural anxiety in the 1920s through 1950s, but only with the combination of medical and law enforcement scholarship and renewed media attention (plus a whole lot of victim self-advocacy and controversy) did the idea of child sexual abuse or sexual assault trauma emerge in society as a discrete syndrome with formalized precursors and sequelae.

The television show Mindhunter is specifically concerned with a fictionalized version of the FBI's Behavioral Sciences division in the 1970s, and it coincides with the forming idea of the "serial killer" as a discrete kind of murderer with a unique psychological makeup. Earlier in the 1970s, an organized repeat killer like Dean Corll or Juan Corona might be called a mass murderer -- but what differentiates an individual like Corll, who sexually tortured and murdered 28 boys over a period of years with the assistance of two juvenile accomplices, from an individual like Charles Whitman, who shot and killed 16 strangers over the course of 96 minutes after murdering his mother and wife? Both crimes might appear psychologically inscrutable, both crimes involved a large number of victims, but they don't have much in common on the surface besides that -- were Whitman and Corll born bad, or had they acquired some biological defect? What else separates men like them from other criminals and from one another? It's from research like the BSU's that modern criminal psychology has developed its differentiations between mass murderers and serial killers, between serial killers and spree killers, between organized and disorganized offenders, and so on.

The characters in the show that express this viewpoint (I assume you mean the discussions of motive and pathology in the first episode that touch on Manson?) are expressing a formulation of new crime that posits a change -- that in the past crimes have been perpetrated by people with straightforward rational motivations, even if their behaviors to fulfill those motivations were not ethical, and now in the 1960s and 1970s this familiar kind of crime has been supplanted by unfamiliar crimes with no easily-apparent motivation. This is much too pat, but it's definitely influenced by law enforcement approaches to criminal motivation as well as cases in Holden's own recent memory that he'd like to inquire into -- cases like the Manson murders or the Boston Strangler killings, or serial killers like Richard Speck and Herbert Mullin. In addition to law enforcement-specific aspects, the general contemporary perception of society in the 1960s and 1970s as more dangerous than any prior time in American history (due to both genuinely shifting crime rates and other factors like urbanization, race and racism, drug culture, the Vietnam War, etc.) is hard to separate from the show's articulation of this shift. In some ways these shifts are perceptible and "real", in other ways they're part of a media narrative of the cruel disillusioned 1970s that existed even at the time the show was set. The modern version of this argument is "what the hell was in the water that there were so many serial killers in the 70s/80s?" -- maybe there were factors that resulted in a spike of serial murder perpetrators in these eras, but that's obscured by a sudden uptick in both law enforcement's ability to identify these cases as distinct from the bulk of other crime, and the media's ability to call these crimes under a single heading of "serial killings", not mass murders, slayings, stranglings, etc.

I would strongly rec John E. Douglas, Robert Ressler and Ann Burgess' works about serial killer psychology and sexual violence, both their separate works and their collaboration in the form of Sexual Homicide: Patterns And Motives. They shed a lot of light on Mindhunter as a show, and also on how Mindhunter has cherry-picked its focus on sex crimes, omitting other kinds of seemingly-inscrutable post-1950s crime like the work of psychologically disturbed would-be assassins or non-sexually-motivated serial killers.

I would also recommend Ginger Strand's wonderful book Killer On The Road for the cultural precursors to this argument related to the US interstate system.

(If you're interested in a variety of historical explanations of what we would now call serial murder I can give you true crime reading recs aplenty. I moonlight on this sub from /r/unresolvedmysteries and /r/mindhunter, so if there's a specific assertion or claim made in a certain episode I'd love to help give in the fact-or-fiction treatment.)