Why do people think Hitler was an idiot simply because he was insane?

No doubt he was one of the most evil men in existence: insane, cruel, paranoid. I just don't understand how people can deny that he was actually a very intelligent leader. I've heard that he was "not intelligent" simply because he led a Holocaust against a people that didn't deserve persecution, and because he locked up some of his relatives in insane asylums: these were products of his insanity, not lack of intelligence.. I've also been called a "sick bastard" for respecting the kind of intelligence the man had. I'm not saying that he is my idol by any means, but he truly was a master manipulator. He knew just what to say to people to get them to follow him, and he was able to pull Germany out of depression from the Weimar Republic. Some of his more foolish military decisions aside (i.e. invading Russia which basically cost him the war) militarily he actually made some decent decisions, even though he was more of a politician than a commander and thusly couldn't control his armies forever. Truth be told, many evil people possess intelligence.