I tried googling "why did christchurch use leetspeak" and didn't get results. I presume it was to get more views from incels and because he found it funny. Not because he was radicalized by the Navy Seal copypasta.

With Peterson and Shapiro I feel like you're trending toward a standard of "anything terrorists like to read, but I don't, should be censored". They're close to as popular as, say, Rachel Maddow, which means almost all of their readers are peaceful.