It certainly would have been more shocking if he were invited in, but I also think that Shosanna wouldn't have shot him unless provoked. She was avoiding that and trying to send him back to the theater, but Fredrick wouldn't have that; Shosanna was running out of time and with his near-violent out-burst, she saw it as an opportunity and a solid reason to take him out, only to regret doing so and finally feel something for him. To take out the entire Thrd Reich in a large fire would be the easiest solution as they could be seen as one faceless mass, but to know Fredrick and have to face him made killing him so difficult for her, I think. Interesting to note that the title of the Ennio Morricone track that played during this scene, Un Amico, translates to A Friend.

The script mentions how Shosanna began to feel for Fredrick (other than the projection booth scene), but after the meeting with Goebbels and then running into Landa at the cafe, it was noted that Fredrick was "Trouble with a capital T and she needs to stay the fuck away from him."

There were other scenes filmed that were cut from the film's release, as Tarantino was wanting to get the film premiered at Cannes and was in a hurry to edit it down. The DVD release will most likely have these scenes restored, such as Donny's backstory and buying his baseball bat (Cloris Leachman appeared in this scene as well) and Shosanna meeting and being taken in by the theater's proprietor, Madame Mimiuex (played by Maggie Cheung).

I'd really like to create the community and I'm currently working on a banner after screencapping Fredrick and Shosanna from the Inglourious Basterds website. It'd also be a safe place for everyone to maturely discuss the relationship between Fredrick and Shosanna, as well as to post any kind of fic and other fan creations.

And speaking of fic, a user on imdb proposed an idea of wherein Fredrick learns who Shosanna really is. She is then murdered by Landa, which prompts Fredrick to join the Basterds and carry out her plan.

Okay, enough rambling! I haven't had this much fun with characters since Adrian Veidt entered my life (I now like to pretend that Hugo Stiglitz is his real father).