Even if he doesn't want to delete these manifestos because of free speech (despite the fact that Null himself says that Kiwi Farms is not a free speech site and bans dozens of users every day for criticizing or disagreeing with him), Null could easily prevent this by editing these posts and locking manifestos under the "personal info" tag to prevent schizos from accessing these manifestos without creating an account that can easily be tracked and banned. Null refuses to do this though because one of Kiwi Farms' main sources of traffic, outside of hosting revenge porn, bestiality, and aiding cybercriminals in swatting and sim-swapping schemes, is aiding far right schizos who want to further radicalize themselves into commiting acts of violence and terrorism.
As much of a role as 764 and the groypers played in influencing the shooter, Null (and to a lesser extent, the Sharty) are the ones who really have blood on their hands.