I think you could have a National Socialist themed army without covering it in swastikas. If someone has to put swastikas and whatnot all over a 40k army I question their motives, it is in't a WWII period game so chances are they are doing it just to shock. If someone has a converted National Socialist themed IG army with appropriate colour scheme and whatnot, sans swastikas, that is a good, subtle nod to the fascist feudalism of the Imperium.
Space Marines, not so much, as they are more like Knights Templar than the SS.
People can be easily offended, and I'm not particularly fond of censoring things just because they are offensive to some. But the fact is National Socialism led to destruction on an unprecedented scale, the human, cultural, architectural and economic costs were immense. Ignoring that just to put swastikas on your plastic toy army smacks of immaturity more than anythign else.