This is a film about the Holocaust. To sugarcoat what happens is the same thing as saying "what happened to the Jews in Nazi-occupied Europe really wasn't so bad." That ain't gonna happen, not with this movie. Spielberg doesn't soft pedal the terror, humiliation, and arbitrary killing that was part of the day-to-day existence for the Jews in Poland.
The violence arrives with sudden force: one pull of the trigger and somebody's dead, blood's on the snow. The ease of it becomes terrifying: how quickly, how casually the Nazis kill. We don't feel any catharsis or release, and the black-and-white cinematography—which lessens the more gratuitous aspects of the death and carnage—has a way of heightening the terror. We feel sick to our stomachs at the sight of it, which is exactly how it should be.
The nudity serves the same purpose. In one scene, the Nazi doctors order the Jews to strip and run around the courtyard. There's nothing titillating or arousing about it; the prisoners are vulnerable and helpless. They're being put through their paces to determine who's healthy enough to work and who will be executed out of hand. And they know it; they're terrified. If you can hardly watch, don't worry; it shows that you're a human being sympathetic to the suffering of others. Unlike some people we could mention.
Spielberg could have toned it down. A number of Holocaust dramas do. But lessening the violence and brutality means lessening the impact on the audience. In fact, the reality was even more unthinkable. Spielberg spared us scenes of thousands of skeletal bodies in mass graves or emaciated corpses being shoveled into ovens. He was making a movie for what he hoped would be a mass market, which gave him some limits to work with. (For comparison, you could watch a documentary film like Night and Fog, which includes newsreel footage. Still, Spielberg told the story that needed to be told and that story deserves an R.