Now, it could be the cause of this tragedy is Satanism and "bad music" but, given the counterfactual nature of causality, it is impossible to tell in a specific case because n=1. In other words, it's impossible to establish causality because confounds and third-factors cannot be ruled out. Correlation of course does not prove causation, so perhaps teenagers who are troubled to begin with tend to be both prone to violence and attracted to Satanism and heavy metal. Disentangling the direction of causality is possible with general studies but, unfortunately, the scientific literature on the media's influence on violent behavior is infested with moral panic, ideology and unnecessary shouting. However, a quick look through the literature turned up two interesting studies. The first, a review of meta-analyses published in The Lancet finds:
Community leader, Pierre Eksteen, who is in charge of a school support network for children, told reporters outside the deserted school grounds that Satanic music was probably the cause of the attack.
"He came here camouflaged as the guys from Slipknot. We know the wrong kind of music, and drugs have bad effects. Young people need to be informed of the effects of bad Satanic music," said Eksteen.
There is consistent evidence that violent imagery in television, film and video, and computer games has substantial short-term effects on arousal, thoughts, and emotions, increasing the likelihood of aggressive or fearful behaviour in younger children, especially in boys. The evidence becomes inconsistent when considering older children and teenagers, and long-term outcomes for all ages. The multifactorial nature of aggression is emphasised, together with the methodological difficulties of showing causation. Nevertheless, a small but significant association is shown in the research, with an effect size that has a substantial effect on public health. By contrast, only weak evidence from correlation studies links media violence directly to crime. (Emphases added).In other words, we know the media has a small but statistically significant short-term influence on aggressive behavior in young children, but there is no consistent signal for long-term behavior or older children. Dealing specifically with the influence of heavy metal, Roberts, Christenson, and Gentile (pdf) conclude:
The best way to phrase the relation is to say that white adolescents who are troubled or at risk gravitate strongly toward the style of music that provides the most support for their view of the world and meets their particular needs: namely, heavy metal (p. 162).Roberts et. al., therefore, conclude that there likely isn't a causal relationship between heavy metal and troublesome behaviors - troubled teens are drawn to heavy metal, heavy metal doesn't turn teens bad.
The literature on the media and violence is truly massive, and the above doesn't come close to being exhaustive, systematic or even representative. Nevertheless, it's clear that the knee-jerk community and media reaction - "Satan!" "Heavy metal!" - is unjustified and irresponsible. The causal relationship (if any) between violence and Satanism or heavy metal is in general unclear and difficult to establish and impossible to determine in a specific instance. While I think he is being somewhat too simplistic, I tend to agree with Ray Hartley from The Times:
Let’s hope that, in their rush to find supernatural causes for this tragedy, the good folk of Krugersdorp don’t forget to examine themselves. For, I fear, the real cause of this tragedy lies closer to home... Perhaps somewhere in that adolescent stream of crap that streams from Slipknot, he found a channel for his rage. Rage that may have come from school or from home or both - who knows. But to argue in all seriousness that this outlet for his real rage was the cause of his rage is facile and short-sighted. It will be a popular theory because it excuses those who responsible for raising this child in a nurturing, caring and protected environment from responsibility for their failings.(Hat tips: Skeptic South Africa and Yet Another Sceptic's Blog. See also: "The devil didn't make him do it").