My bet; porn.
My bet: very bad childhood, or some sort of biological brain abnormality.
He very well may have looked at pornography, even sick pornography, but that would have been caused by his being a sicko in the first place, the pr0n would not have turned him bad.
Plenty of kids look at pr0n without committing rapes and murders.
Too bad that we seldom here what these murdering perverts did in their pastime. . .what kind of music they favored; books read or magazines. . .or internet pastimes. . .or even what video game was their favorite etc. I really would like to know; would like to know more of what these people find their pleasure in.
Not to blame the above in any single sense of course; but would be nice to know just how 'off the mark' or on. . .they might have been re their private lives.
And just how off. . .or on the mark; we might be, should we make assumptions.
Porn is a reasonable assumption; it is surely corrosive in it's impact on young people. . .and old as well.
Just want to add again, that do not see these choices in any 'single' sense as causation for criminal behavior. . .but for the already behavior-challenged kid; there are a lot of 'bad' choices available.