I think there are two questions here that you guys fail to separate.
1. Did the Nazis deserve to die (within the movie universe)?
2. Why do we as an audience, ENJOY watching them die?
but not all of us do enjoy watching them die, or get joy out of it. if you get that joy, it is a great question for you, i suppose. my engagement with the film's violence is quite different.
The theater I was in exploded in cheer during the climax, and I live in Ithaca NY, the most peacenik town in America. I am pretty sure Tarantino intended for us to cheer for the bad guys demise, the scene is clearly filmed in a very dramatic and exploitative fashion, nowhere is any pathos shown to the Nazeees. How else can you possibly react to that scene? Shed a single tear for poor Adolf?
on your first question, i don't think it is obvious in the movie universe that the nazis deserved to die. the fact that we are introduced to Aldo giving his venomous speech about killing nazis immediately after the venomous rhetoric we are previously introduced to from the nazis is no coincidence, these sentiments are clearly meant to echo. this is meant to complicate the assumption that (uniformed) nazis deserve to die, that the work of the Basterds is honorable
Disagree completely. Aldo's speech wasn't venomous at all, it was clearly meant to be humorous or atleast tongue in cheek (killin nazeees). The showing of Nazi barbarity before the speech is not meant to show any similarity, its how all revenge films are structured.
Scene 1: A terrible crime by villain
Scene 2: Hero vows revenge
Rest of Movie: Hero gets revenge
Scene 1 makes us hate Nazis all over again, scene 2 makes us giddy for the revenge to come, the ending is the culmination of that promise of vengeance. Again, nowhere is any equivalence drawn between the heroes and the Nazis.