The best reason not to release the manifesto is to not encourage other would-be mass killers with manifestos.
Second reason, to spare families of the victims.
Weighed against that is the possibility of understanding what makes someone turn murderous in that way, and preventing that evil turn, or of recognizing murderousness in time to stop it.
“spare families of the victims”
That excuse has been used to oppress the masses for centuries—it is just not going to cut it any more.
And most importantly — to show how the left’s relentless racist messaging about “white privilege” can inspire deadly violence.