When I state my beliefs at length, it’s a manifesto.
When you do it, it’s a rant.
I periodically (and often) plant my little feets and issue a screed about how I see things, declarative. You can dissent, you can disparage, but at least, what the fuck, I said how I felt. Totally manifestos. I’m practically ManifestosRUs, come to think of it. I’m willfully opinionated and I think everyone should be entitled to my opinion. Whether they like it or not. Any questions?
NPR seems to agree with you.
From that article:
Webster’s New World Dictionary defines “Manifesto” as, “a public declaration of motives and intentions, as by a political party or by an avant-garde movement” So to refer to the shooter’s document as a “manifesto”, in my opinion, implicitly takes the twisted racist and anti-Semitic rantings that comprise it, far more seriously than they deserve to be taken and gives it a level of credibility that furthers the shooter’s aims.
A screed seems more appropriate:
A screed, in contrast, is a long and tedious piece of writing. In the 18th century, it was possible for a screed to be dull and inoffensive – you could have “a screed on poetry,” for example. These days, the word suggests something that is full of anger and ranting. Screed is related to shred and originally referred to a scrap of cloth or paper. It probably acquired its negative connotations because it sounds like “screech” or “scream.” - SOURCE