This is not remotely the same thing as dropping nerve gas on apartment buildings. Spicer did not try to compare Assad's brutality to Hitler's. He was referring to specific battlefield use of chemical weapons Pointing this out in no way diminished the horrors of the concentration camps or Hitler's final solution and no one would have been outraged if President Obama's spokesman had made a similar statement. It's just gotcha politics.
So why the controversy? It's because the Nazis used gas chambers to kill Jews and other minorities.
While making a point about the brutality of the Syrian government, White House spokesman Sean Spicer pointed out that even Hitler didn't use chemical weapons. This should have been an uncontroversial statement since it is true. World War One saw wide-spread use of mustard and chlorine gas warfare. It was widely anticipated that Hitler would order the V2s fired at London to be equipped with similar payloads and images of the Blitz show Londoners in gas masks in preparation for that.Similarly, battlefield troops across Europe were equipped with gas masks which they never needed.