Julius Rosemeyer is the high-ranking Wehrmacht officer sent from Berlin to assist castle commander Colonel Kramer in interrogating the captured General Carnaby. He's an older man an a bit more of a reasonable fellow than the icy young SS officer.
When the castle's dining room turns into a shooting gallery following the intrusion of Major von Hapen, things happen in quick succession. Von Hapen dies, and then Kramer goes for his telephone.
While Kramer is doing that, Rosemeyer, showing us how he got his rank, through quick thinking, goes for Smith's discarded PPK, which von Hapen had forced him to throw down upon entering moments ago.
He grabs it and brings it to bear on Smith and Schaffer. Unfortunately for him, the younger Schaffer is quicker. Even as Rosemeyer was getting the gun, the American was shooting and killing Colonel Kramer to stop him from calling for help. Now, he spins and aims at the General.
Before Rosemeyer can shoot, Schaffer beats him to it, hitting him in the gut. Proving to ever the tough old soldier, Rosemeyer doesn't die instantly or even show much pain. He simply grunts and makes a face suggesting mild discomfort, tries to remain standing and get a shot off before dying, but ultimately succumbs and drops the gun, then keels over.
Like Kramer's death, Rosemeyer's is somewhat of a letdown for what is supposedly one of the movie's principal villains. In fact, all three of the film's primary Nazi heavies die in the dining room scene, leaving us with just the cowardly, ineffectual British traitors, who mostly prove too pathetic to really be a threat.