This was one fucked up Australian, from a country of over 30 million; and the first Aussie to commit a mass shooting since 1996.
I do not think it's right to place the blame on a bunch of random people saying things. They didn't murder all those people. Speech is not violence. Violence is violence, and that one guy decided to take it off the computer, hop across the pond, somehow get a ton of firearms in a country where they are heavily restricted, and then shoot up a bunch of innocent civilians.
There are millions of other Aussies and Kiwis who probably chat on the same networks he was on, and never take up violence. They may speak up against things they think are wrong or that they don't like, either online or in public spaces, but there is a big difference between speech/saying things and murder.