Both the metaphor and its literal application only make sense if "cannot" and "does not" means "never", and they really don't.
While I'd never fear the mind of a man who literally is in a coma and doesnt think at all, I'd have plenty of reason to fear the mind of a man whose ability to think is merely limited. He can be a stupid moral reasoner and a clever killer at the same time.
"As I fear not a child with a weapon he cannot lift, I will never fear the mind of a man who does not think.’”
Words of Radiance, Brandon Sanderson, page 795