I seriously bet it is KissManga. Is there a way of doxing their administrative staff? If you have real life information aren't their people willing to do physical violence against them for just a payment in cryptocurrency? Hell, if they were near me I'd do the violence myself. There is no way they'd ever connect a seemingly random act of violence to DDoS attack on a rival manga site.
We basically have no legal recourse, so why shouldn't we resort to violence? Some unknown entity out their is perpetrating a malicious and illegal assault on collective interests. We have to respond in some way or they win. With the way things are going they'll be able to inflict their violence consequence free. The only rational thing to do is to respond with a more extreme action otherwise their assault will continue.
Physical violence may seem, but I can't stand people who think they are entitled to do whatever disgusting want with no fear of consequences. I don't personally a problem with ending someone's life over DDoS attack on manga site. Chances are perpetrator is a truly evil person and their existence needs to be erased. They're the sort that would destroy people's live if they held an actual power. Their existence likely causes pain to everyone around them. They are of no use to anyone. From a Utilitarian standpoint it would really be better to kill them before they take their entitled attitude into the real world and cause problems.