Billings later recalled that Kennedy was “completely consumed by his interest for the Hitler movement” during their trip.
Some context for this; JFK was 20 years old at the time and was likely heavily influenced by his father who was a fairly blatant supporter of Nazi Germany. And while his views seem shocking now, it's important to remember that at the time they were conventional wisdom. There was a sizable strain of admiration for fascism in liberal circles extending into the FDR administration. The sense was that fascists were capable of getting things done.
England, in many American circles, had come to seem like Israel today, a country that dragged the United States into wars. While Nazi Germany, like the Muslims today, was seen as misunderstood and a victim of history. Kennedy's observations would not have been unusual even for many liberal visitors older than him who came away from Germany with the same impression that the country was on the way up and that it just needed to be left alone.
After a visit to the river Rhine in 1937, Kennedy wrote: “Very beautiful, because there are many castles along the route. The towns are all charming which shows that the Nordic races appear to be definitely superior to their Latin counterparts. The Germans are really too good – that’s why people conspire against them – they do it to protect themselves.”
A fortnight earlier, Kennedy, who was touring with his friend Lem Billings, wrote in his diary: “I have come to the conclusion that fascism is right for Germany and Italy. What are the evils of fascism compared to communism?” Billings later recalled that Kennedy was “completely consumed by his interest for the Hitler movement” during their trip.
But even after Germany’s defeat in 1945, when the Holocaust was common knowledge, JFK appears to have retained an extraordinary fascination for Hitler. By then a naval officer he accompanied the US Secretary of the Navy, James Forrestal, on a tour of Germany that August.
After visiting Hitler’s bomb-damaged Bavarian Berghof residence and his Eagle’s Nest mountain retreat, Kennedy noted in his diary: “Anyone who has visited these places can imagine how in a few years, Hitler will emerge from the hate that now surrounds him and come to be regarded as one of the most significant figures ever to have lived.” He adds: “There was something mysterious about the way he lived and died which will outlive him and continue to flourish. He was made of the stuff of legends.”
By that point, JFK was no longer twenty and there's less excuse for it. But let's look at mainstream media reports from the 1930s, which weren't filed by bored 20-year-olds.
Nazi Drive on Jews Under Control Now: US Investigation Shows No Cause for Protest (AP)
March 26, 1933
"A reply has now been received indicating that whereas there was for a short time considerable physical mistreatment of Jews, this phase may be considered virtually terminated. There was also some picketing of Jewish merchandising stores and instances of professional discrimination.
"These manifestations were viewed with serious concern by the German government. Hitler in his capacity as leader of the Nazi party issued an order calling upon his followers to maintain law and order, to avoid molesting foreigners, disrupting trade and to avoid the creation of potentially embarrassing international incidents."
Fortunately Hitler turned out to be a moderate Nazi and that was the end of it.