> must beware of anyone who is stupid and diligent -- he must not be entrusted with any responsibility because he will always cause only mischief
Well, hard-working but not clever people will do useless work and create extra work overloading everyone else. BUT, this happens when they don't have good management and leadership above them, to prioritize and handle them bite-sized pieces, and to set their frame of mind to focus on "results in the bigger picture".
Probably putting "clever and lazy" and leadership roles is the cause for bad management and leadership which makes his "stupid and diligent" underperform in ways that drag everyone down.
Sayin' this as a "clever and lazy" person myself - I know that if I don't have someone I am responsible to (doesn't matter if it's a managers, partner, peer etc.) that is truly hard working and diligent is a recipe for total disaster. I'd rather be accountable to people way less creative, and even lower both IQ and EQ, as long as they come with a solid work ethic and focus to detail and they manage to drag me in this direction to! If the world would have only of people like myself in all leadership positions, nothing would ever get done, and the "diligent / hard-working" people will probably go crazy and either kill themselves or start wars.