Violent media can be an influence on such a person though as it often normalizes atrocious acts.
That is the worry. Not that violent media creates such people, but that it might trigger an expression of those latent tendencies into actual violent behaviour.
One of the other themes I read a while ago was about killer couples (and also a shift to risk). The ideas were that
a) the murderous intent of a couple could be greater than the sum of their separate murderous intents. That is to say there were a number of couples male+male, female+male etc who went on to murder when neither of them as an individual would have expressed that tendency. That as a pair, they found a legitimacy or a "courage" to comit vile acts and cross a threshold that neither woudl have individually.
b) the shift to risk in group behaviours. The larger a group - typically of young men - the more they were likely to exhibt extremes of risky behaviour eg tendencies to group or mob violence (or racial abuse on a paris underground).
The great fear of the internet is that it effectively creates such huge groupings that it could possibly triggers an expression of vile behaviours by people who would have previously kept such thoughts locked away in their minds. Studying the Nazis for A'level was an instructive and scary exercise in seeing how normal people, in the absence of any moral brake or compass drifted into unspeakable evil. Lord of the Flies is another example of how fragile and artificial civilisation is.
It is hard to research the effect of what is being put out there - are we murdering more than we used to? I don't know, but certainly it is feasible to witness it more now. For example with a national news figure reporting on how he felt he "had to watch" a 20 minute ISIS video of a man being burned alive, and yet three school girls can still be comvinced to travel across europe to join that organisation.
WTF is basically what I am saying.
The value and meaning of humanity is under more pressure than it has been since the Romans discovered a novel feeding regime for lions and a way to curb religious extremism at the same time.