We cannot rely upon this to be the actual words of the shooter. Anders Behring Breivik wrote a manifesto (using the pseudonym Andrew Berwick). He went on to commit a mass shooting in Norway.

In response, Operation UnManifest was undertaken:

https://pastebin.com/DktSNbme

This could easily be just the kind of propaganda all the hate speech and calls to violence in the past couple of years has been about. We need to know who is behind it! People are dying as a result of efforts to essentially shut down free speech on the Internet.

Look how many times the author used the words "browsing" and "found". It blames the Internet for the shooting.

It is reasonable to look for agents provocateur.