There is a perfectly good reason not to release. If this guy shot up 500 people to get some manifesto published, or at least some letter, then in respect to all his victims and their loved ones, do not give him his wish.
Then it would be pretty simple for them to just say "it's a manifesto but we are not going to release it and promote copy cat crimes and to respect the victims". They are not doing that at all are they?
This is exactly what I have been saying from the get go. We must demand these investigators stop working in the shadows. This secrecy is not being done to protect the people it being done to protect the political narrative. Remember, THEY WORK FOR US and we do not have to accept "it's an ongoing investigation" as an answer. Unless giving certain information puts someones life directly in danger we the people must INSIST they answer our questions and work with complete transparency.
There are several perfectly good reason to publish.
Victims families deserve to know why their loved ones were killed not spend the rest of their lives in conjecture. Americans deserve to know exactly which group did this so they can be vilified so the group does not grow. The KKK used to kill and the public attitude turned on them until they became a disgusting joke, vilified at every turn, keeping people away except for a few loons.
Lastly, in todays atmosphere where we are becoming stratified we need to see the real motives and actions of groups so they end before they cause the civil war they want.