It's possible even likely that he had some mental issues. However not to the extent of diminishing his responsibility. He knew what he was doing. Yes he thought it was right but he knew how it would effect people. That he liked those effects does not diminish his responsibility. If it did we'd have to hold nobody responsible for anything even the good people do and then the world we live in would be empty and meaningless and out of meaninglessness springs boredom and out of boredom springs chaos. For this reason Hitler and others like him need to be held responsible and thought of as monsters.
Although we 'Normal' people would consider his dictatorship and racist views as immoral (subjectively of course) that is merely our opinion. Those things are the only reason for arguing that he was insane and they are simply the other side of the coin.
To call Hitler insane you would have to call the millions of people who followed him insane as well and the probability of that many people having the same mental illness the same way in the same place... Well that idea is a bit insane.
It depends on what kind of person you are. Hitler genuinely thought he was doing the right thing for his country because it was the way he was raised. Believe it or not He was close to a normal person. Its like asking if the American people were mentally insane for doing almost exactly the same thing to African Americans. Hitler flirted with girls, he had friends. The only thing why people thought hitler was insane was because he was the figure head of the whole organization. He made the plans, he made the army, he made the motto, but yet again i restate that hitler flirted with girls and went out with friends for drinks. Its all about perspective.