This is important to remember:
Well, I guess we should never assume malice when ignorance remains a possibility.
Which is of course a softened version of Heinlein’s Razor:
Never attribute to malice that which can be adequately explained by stupidity, but don't rule out malice.
We are left with the difficult task of distinguishing these three, the additional category of intentional ignorance, and combinations of the four. Then there is the harder problem of dealing with the varieties of intentional ignorance.