To quote Wikipedia: "The root of 'monstrum' is 'monere'—which does not only mean to warn, but also to instruct, and forms the basis of the modern English demonstrate. Thus, the monster is also a sign or instruction. This benign interpretation was proposed by Saint Augustine, who did not see the monster as inherently evil, but as part of the natural design of the world, a kind-of deliberate category error."
I had know it for years, but really started thinking about it after Anders Breivik's rampage. What he did was a warning, not against Muslims (soon enough they will be swimming in terror back to their homelands) but against leftists destroying Norway. He took out the younger generation before they could grow up and finish destroying his country.
Now we have Dylann Roof, who shot a bunch of harmless black people instead of taking out gangbangers.
He is of course a monster - but what was he warning against? The coming race war, which has already arrived.
All monsters feel humiliated (watch Frankenstein sometime) and humiliation is often followed by revenge (Roof said something about all the white women being raped by blacks).
They are alienated, with no meaning or purpose or community to their lives. Lacking those things is one of the best ways to create a monster. Frankenstein, for example, is so popular because he is a benign monster who becomes alienated through rejection and humilation and so seeks revenge.
Just look at the first recorded murder - Cain humiliated by God's rejection of his sacrifice, so he blamed it on his brother and murdered him. Humiliation followed by revenge.
Breivik felt humiliated watching leftists destroying his country, so he took out the younger generation. Roof, too, obviously felt humiliated and helpless, so he took matters into his own hands.
Such monsters always feel justified in what they have done, because in their minds it's self-defense. They feel they are the ones who have been attacked.