My understanding is that Luigi did not publish the manifesto, and that it was discovered by others later. If that’s true, then the manifesto itself is not particularly relevant to anything criminal. The message on the bullets could be considered relevant, but I don’t see how that alone would be proof of intent to terrorize.
You can certainly interpret the killing that way, but there are many other reasonable interpretations, and to get a conviction you need proof beyond a reasonable doubt. Do we have a quote of him saying before the murder or publishing at any time something indicating that he was killing this guy to send a message to all the evil m************ who act just like him? If we do, your conclusion is warranted. If we don’t, your conclusion is speculation.
Let me give you a parallel. Imagine someone cuts me off in traffic and I pull out a gun and I shoot them. Am I terrorizing other bad drivers? Probably not. Probably I’m a psychopath dealing with road rage in a terrible fashion. In other words, the fact that other people can draw conclusions about similar behavior does not in itself make my actions threatening to them in any way.