Did Uri turn out to be a Nazi and shoot Misha in Jerry Spinelli's Milkweed?
I can't find any anaysis of Uri other than leader of the boys. There is nothing to explain what he was doing working in the hotel and then wearing a Nazi uniform at the end.
Uri had been a partisan, who had ultimately helped lead, or at least had been involved in, a revolt by the Jews in the Warsaw ghetto against the Nazis. The uprising, which took place in April and May of 1943, was unsuccessful, but was considered to be a great act of courage on the part of the grossly outnumbered and poorly supplied partisans, and brought hope to the Jews, calling "all the world's attention" to their plight.
In Chapter 43, Misha explains,
"...I understood at last what Uri had done and what he had saved me from. I understood that the Uri I knew - the real Uri - was not the one the Nazis knew. I smiled to think of him on the last day, once again in his own clothes, shaking his fist at the oncoming tanks, his red hair flying..."
It was Uri's red hair and fair complexion that allowed him to pass as a non-Jew. During the time the Jews of Warsaw were confined to the ghetto, he had been living on the "outside," working at a hotel frequented by occupation troops and ultimately becoming a Nazi soldier himself. Uri's actions were clandestine; his heart was with his people, the Jews, and he circulated among the Nazis to gain information for the underground.
Uri was a Nazi soldier when Misha saw him last, and he did indeed shoot the boy, but not to kill; Uri had his gun "point[ed] between [Misha's] eyes," but hits him in the ear instead. By wounding Misha and rendering him unconscious so that the Nazis would have thought he was dead, Uri had saved his friend from being put on the train to the concentration camps, where he would have been killed in the ovens. Appearances to the contrary, Uri had saved Misha's life (Chapters 39 and 43).
Remember, it was very common for German boys to be in uniform and work with adult German soldiers. Citizens feared them despite their ages. So Uri was kind of a double agent - pretending to be a Nazi, but trying to help his Jewish friends.