I know how difficult public speaking is. I'm relunctant to jump on someone's case for making a public gaffe. I don't think I could do a good job as White House press secretary. But Sean Spicer can't do a good job either.
A good rule of public speaking is to be mindful of Godwin's Law and avoid comparing an adversary to Hitler or Nazis. To be sure, there are bona fide Nazis out there who prefer to be called National Socialists. None of them are running a country right now. If a Nazi or worse does rise to power somewhere, we need to save our best rhetorical ammunition for the occasion.
And when you do decide to open fire with a worse-than-Hilter comparison, make sure that you know what you are talking about. The Nazis are notorious for killing Jews with all sorts of methods, including poison gas. Some of these Jews were German citizens by birth. And the Nazis just didn't murder their own people. They even murdered other Nazis.
Perhaps Sean Spicer was trying to say that Hitler refrained from using chemical weapons on the battlefield. I still fail to see how it's more moral to use Zyclon B on a crowded room of naked prisoners than it is to use against infantry on the Eastern Front.
Which brings us to one more point. Russia has suffered way more from Hitler's atrocities than America has. The Siege of Leningrad makes 9/11 look like a trip to Disneyland. Hitler ordered the destruction of the very city that Vladimir Putin was born in. It would be very hard to convince Putin that Assad can top that level of evil.