The main difference is I suppose is that he supported systematic mass extermination, unless you are of the view that he knew little. However even this doesn't place him any worse off than Stalin.
If we approach systematic mass extermination from Hitler's perspective, then it was simply the business at hand of carrying out intended foreign policy, in this case clearing living space for German expansion. If we submit to his view of the Slavic peoples then it isn't viewed as a holocaust. However, it's a pretty
big 'if' of course. German, English, Dutch, French settlers (amongst others) moved to a land where they gradually displaced the indigenous peoples by force, both directly and indirectly. What they did was essentially what Hitler did to the peoples of Poland, the Ukraine etc. The major difference was the scale that Hitler carried things out on and that it was German foreign policy. However, it was pretty much the same actions with the same results.
It is interesting why we view Hitler as a tyrant and Alexander the Great as a Hero, didn't the latter have a whole city crucified? I suppose the passage of time and the victors allow people to judge differently.
I don't know about Alexander but I do know that Ghengis Khan would burn to the ground (and kill every living thing within) any cities that did not surrender to him. Quite often cities refused to surrender, at least initially. Clearly, these are war crimes on a massive scale yet because this happened some 800 years ago Khan is not viewed with such distaste as Hitler. I wonder if in 800 years time the passage of time will merely mention Hitler as a great and bloodthirsty conqueror like the great Khans and not as an inhuman monster.
Perhaps the main military criticism of Hitler was the blatant way he tore up the Nazi Soviet pact and attacked the Soviet Union. Even Stalin didn't expect this and he must have understood the evil mind.
I am now of the opinion that in principle, Operation Barbarossa was sound. There is some evidence that Stalin was planning an attack of his own, just not in 1941. The Soviet tank armies were going through massive reorganisation in 1941 and some speculate that Stalin planned to make war on Hitler in 1942. Hitler's own planners had advised that German superiority in tactics and in some areas of equipment would disappear by mid 1943. Thus an attack in 1941 when the German armies still had momentum and tactical superiority makes some sense. Where it fell down was in its 6 week delay, its faulty planning regarding the Soviet armies and terrain and Hitler's indecision regarding objectives, e.g. switching the German
schwerpunkt from Moscow to Kiev in August 1941.