There was clearly one murder victim involved in this story, and that's a far more significant crime than "trolling people" or "trolling trolls who troll people".
Of course the killer has committed the worst crime, no doubt about that. But I think we are being conditioned into extremely fast blame games sometimes. These things wouldn't happen in a more evolved society. When is society to blame?
Unless I missed something, the victim noticed that the killer was harassing people online, called him a dummy, and complained to Hatena blog to have them shut down his accounts. I just do not see any sense in which it's meaningfully true that they're "both victims." From what I can see here, "Hagex" was basically a lite Brian Krebs.
Actually if you're looking for something to blame, people are it. There isn't a society to blame. That's an abstract concept only useful if you want to sabotage your ability to think about stuff.
No, you can't think about social dynamics by merely considering people as atoms in a vacuum. Well, you can, but your suppositions will have an attenuated relationship to reality.
The only part of "society" that the guy interacted with were people he interacted with. The person who most directly caused him to murder was the victim.
You're not going to be able to look at some abstract generality like "society" and come to some conclusion about why a specific murder happened. It makes just as much sense to blame "biology" for the crime.
It is an almost willfully blind perspective to pretend like societal mores, practices, beliefs, hell even language, don't exist or are irrelevant. People aren't formed in a vacuum.
