I heard an interesting interview on this point with David Wilson, an Emeritus Professor of Crimininology. In his view this image, with the wild staring eyes, is deliberately contrived. It's consistent with Rudakabana's behaviour in court: the shouting, the attempt to control proceedings, the strong admonition from the Judge that he was the one in charge here not Rudakabana. In short, he wants to shock. He wants people to be repulsed by the image and to say he looks evil. For status, apparently.
The 'turning bad when they reach 13' idea Wilson thinks stems from not having the status they believe they deserve. The word 'incel' was carefully avoided, but I believe the victim who claimed he deliberately targeted women and girls was onto something. The the hideous online content Rudakabana was consuming also draws inevitable comparisons with the two teenagers who murdered Brianna Ghey, and who were also viewing visceral, violent material online. The girl was the main driver and like Rudakabana, was obsessed with extreme violence.
Wilson's view on the question of whether viewing this kind of content fuels violence is interesting. He thinks not. Statistically, violent crime is apparently falling globally. The same kind of panic has been ongoing since changes in the print, photography and cinema ages from the Victorians onward altered the way we consume material. The same arguments happened again with video nasties in the James Bulger case.
The mugshot of Eddie Radcliffe, one of Brianna Ghey's killers, struck me in the same sort of way as Rudakabana's. But where his eyes look feral, wild and staring, Radcliffe's look glassy and completely empty, as though there's nothing behind them at all. Clearly, what chills people is the knowledge of what these killers did, and these images in some way fit that revulsion. I agree with the PP above that labelling and othering 'evil' as in some way detached from ourselves is a natural human response, but also potentially dangerous.
Some interesting responses upthread.